Duramax L5P Engine Problems

The 3 Most Common Duramax L5P Engine Problems

Jake Mayock

Meet Jake

Jake is a founder of 8020 Media and has been creating automotive content online since 2017. He has been the lead writer for Chevy Trucks and has transformed it from the old and outdated site it was into what it is today. Jake creates a ton of GM related content for the 8020 Media YouTube channel and specializes in Duramax and Vortec information but has a wealth of knowledge across all GM cars and engines. Jake believes the L5P is the best diesel on the market today.

The Duramax L5P was introduced in 2017 as a replacement for the LML. While the engine uses the same 6.6L design, many major engine components were completely refreshed in order to increase power as well as engine reliability.

The L5P has made great reliability improvements and is, in my opinion, the most reliable diesel engine being produced today. However, it does suffer a few common problems such as MAP sensor failure, a weak transmission, and fuel injector failure.

I’m going to discuss the important reliability changes that were made to the L5P and then jump into each of the common problems in detail.

Duramax LML vs. L5P Differences

Despite the same 6.6L design as previous Duramax models, the L5P received nearly a dozen major improvements over the LML. Outside of the major items covered below, the L5P also received: a strengthened cylinder block, improved glow plugs, a new camshaft design, a high capacity engine oil cooler, an upgraded EGR system, and a stronger transmission with a new torque converter.

Understanding the differences and the new equipment on the L5P is important for understanding the reliability improvements made over the previous Duramax version.

Borg Warner Turbocharger

Previous Duramax engines had used a Garrett turbocharger which has been replaced with a Borg Warner turbocharger. Previous turbos were variable vane turbos (VVT) which were controlled and actuated via oil pressure. The new Borg Warner turbo is electronically controller and actuated and uses a variable geometry design (VGT). The VGT design is a lot more efficient and reliable.

Denso HP4 in Favor of Bosch CP4.2

The Bosch CP4.2 was a wildly common problem in the LML. Bad fuel would deteriorate the internals of the fuel pump and then send small metal shavings throughout the whole fuel system. This would cause a chain reaction that would then essentially destroy the whole fuel system and lead to $10,000+ of repairs.

Fortunately, in the L5P the CP4.2 was ditched for a Denso HP4 injection pump. The HP4 is capable of handling higher fuel pressures which allows for smaller injectors without losing power potential. The stock HP4 has is capable of handling 800+ horsepower, although upgraded injectors will be required to hit these numbers.

Denso Fuel Injectors

Alongside ditching the Bosch injection pump, the old Bosch fuel injectors were also left behind in favor of new Denso injectors. The new Denso injectors flow approximately 20% more fuel than the previous Bosch injectors and also use a solenoid-style design, replacing the piezo design.

New Intake & Hood Scoop

A hood scoop was added to the exterior design of the L5P. With the new design, approximately 60% of air is provided via the hood scoop while the remaining 40% is sucked in via a traditional intake system that sits within the front fender.

The hood scoop draws in cooler air and provides a ram air effect which improves turbo efficiency and reduces lag, resulting in overall better performance.

Duramax L5P Engine Problems

  1. MAP Sensor Failure
  2. Allison 1000 Transmission
  3. Fuel Injector Failure (#4 Injector)

Finding 3 problems with the L5P was difficult. While these engines are still relatively new, they seem to be extremely reliable so far with very few “common” problems that haven’t been solved or addressed.

If you would rather consume this content via a video, check out our GM L5P Duramax Common Problems video below:

1. MAP Sensor Failure

The L5P uses a manifold absolute pressure (MAP) sensor inside the intake manifold. The MAP sensor is responsible for sending the ECU vacuum and positive air pressure readings which are used to determine how much fuel should be sent to the engine for combustion. Additionally, the MAP sensor is critical in determining when ignition should occur within turbocharged engines.

Like any sensor, the MAP sensor is prone to becoming clogged or gunked up which can affect the readings being sent to the ECU. On the Duramax L5P, the MAP sensor is known to become clogged with soot or grime and either fail or send incorrect readings to the engines computer.

When a MAP sensor fails, inaccurate vacuum and air pressure readings are sent to the computer which then affects the amount of fuel that is being sent into the engine for combustion. Overall, a bad MAP sensor will cause poor performance and can lead to catastrophic engine damage if it’s not fixed in a timely manner.

Symptoms of a Bad MAP Sensor

  • Engine misfires
  • Rich/lean air-to-fuel ratios (AFRs)
  • Check engine light
  • Lack of power and overall poor performance
  • Rough idling
  • Engine stalling, hesitation, etc.

Replacement Options

The MAP sensor on the L5P requires constant cleaning. If you have a bad MAP sensor, installing a new sensor will only reset the clock. Eventually the new sensor will become clogged and fail again.

There are generally two options to prevent the MAP sensor from clogging:

  1. Constantly clean the MAP sensor with MAF Sensor Cleaner
  2. Install a MAP Spacer

A MAP Spacer reduces the amount of soot buildup by spacing it away from the EGR stream which is what predominantly causes the buildup.

2. Allison 1000 Power Limitations

The 6-speed automatic Allison 1000 transmission on the L5P is the strongest Duramax transmission yet. With a new torque converter and several other upgrades, the transmission is stout and shouldn’t cause any issues on a stock L5P. With that being said, this isn’t really a “common problem” with the L5P. However, we did want to note that there are some limitations to the transmission for those looking to make big power.

With the stock fueling system, turbos, and internals capable of handling up to 600whp without any modifications, it can be tempting to throw some additional power at your L5P. Unfortunately, the biggest limitation to big power is the Allison 1000 transmission. The transmission is generally capable of handling ~500whp without many issues, but once you start pushing past those levels, you will run into some expensive transmission upgrades.

For those with heavy modifications and a heavy foot, some upgrades such as an upgraded trans cooler, a multi-disc torque converter, hydraulic upgrades, and improved friction materials are likely necessary to avoid expensive transmission rebuild costs.

3. Fuel Injector Failure

While not the most common problem, the #4 fuel injector specifically is known to be problematic. The injector connector for the #4 injector is known to go bad and send the L5P into limp mode with a P0204 engine code. The engine becomes very shaky and various additional engine lights will appear such as Stabilitrak and ABS.

While the #4 injector itself can fail as well, a lot of people are reporting issues with the connector itself and not the injector. It seems the trick so far is to either use an LBZ connector or get a gold plated connector, both of which make a better connection.

Additionally, there was a recall for fuel injector calibration (recall N182179090) in February, 2019. While this recall isn’t tied to the #4 injector, it is worthwhile making sure you have this completed to prevent any future injector issues.

Symptoms of Fuel Injector Failure

  • Engine codes P0204, P020D, P0300
  • Limp mode, ABS and Stabilitrak lights come on
  • Engine misfires, rough idling
  • Shaky engine, poor performance

Duramax L5P Reliability

The L5P is considered the most reliable post-emissions Duramax made to date. With the removal of the CP4.2 fuel injection pump and various improvements made to the block, pistons, and other internals, the engine itself is extremely capable and reliable. The turbo itself is capable of handling up to 600whp, and the stock fueling system can handle up to 800whp, making this a very tuner-friendly engine.

With the Allison 1000 upgrades the transmission won’t cause you any issues on a stock L5P, although it may become problematic under heavy modification. So far there don’t appear to be any major issues with the emissions components (EGR, DPF, SCR, etc.) although DEF fluid may require flushing a bit more frequently.

With proper maintenance, the L5P should be able to easily clear the 350,000 mile mark, with some owners claiming they have had virtually zero issues through the 400,000 and 500,000 mile mark.

Overall, these engines have proved to be extremely reliable.

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  1. Very good information😁. My 2021 GMC 2500 HD 6.6 Duramax Diesel has 3,000 miles on it and has not gone into regeneration nor has asked me to continue driving while a regeneration is taking place.
    I have read in the owners manual supplement to the diesel that’s regeneration is suppose to happen with every tank of fuel. I have not seen this. What is this soot mass sensor calibrated for regeneration. Past information 2018-2020 says 44 gram soot. Does not talk about the 2021 L5P. Should I be worried about a proper regeneration happening? Also I have noticed my oil gauge is on the fritz. Cold starting in the garage here in Florida the oil gauge registers 60-80 PSIG when starting. Allowed the engine to warm up a few minutes and travel for a few miles before it drops to 40 PSIG. Should I be worried. Is the breaking oil causing this to happen.

    1. I have a 2017 and i only get a “regeneration” indication if i haven’t driven very fast or very long in awhile. As long as my normal driving takes me over 40 mph for a decent amount of time i don’t get the instruction to continue driving. Hope that helps

    2. How do you know that your truck has not done a regeneration? Without my Banks iDash I would have no idea that my 21 3500HD is doing a regeneration.
      Your oil pressure sounds perfectly normal.

      1. I run a banks idash from 100000 k – 220000k. Had def wiring issues, deleted egr, dpf, and def, runs great now. Tons of power still have pedal monster and idash integrated. Keeping eye on get for turbo, Gale banks says keep it under 1500 F

  2. I have a 2020 Chevy 2500 hd diesel

    I got the truck at 115 miles and currently I have 55k miles. Had no major issue so far, I love it very much, the power is amazing, transmission is smooth.

    The only issue I have is the infotainment screen shuts off sometimes

    And my def glitched out before , the dealer did a update in computer for free and 20k miles later no issues.

    All in all my truck is amazing and I’m glad I got it.

    1. My 2019 had the same infotainment issue, dealer had to replace a module in the system. They first replaced screen didn’t fix it. Then replaced one module didn’t fix it. Then replaced a different module problem solved.

  3. I have a 2018 that I had absolutely no complaints with for the first 80,000 miles. For a 4×4 crew cab dually, it consistently got 17mpg empty and between 9.5-11 towing depending on load and wind. These are actually calculated readings, not dash “estimates” which are always incorrect. Down to the issue which literally happened overnight, and after a reflash, it lost about 20% in power, dropped to 13mpg empty and 6mpg towing the same toy hauler it used to get 10-11mpg, and will drain the DEF tank in under 1000 miles while towing where a tank of DEF would regularly last 3500-5000 miles in the past. After 3 dealerships agreeing that fuel and DEF consumption is excessive and numerous loaded test drives, GM came back with all is normal. This is a vehicle rated for towing 35,000 pounds and it’s is struggling hard to tow a 14,000# toy hauler. Bottom line is it is still under power train warranty and it can not do what it was designed, built, and bought to do. To add to this, one dealer replaced the DPF with a new one because it commanded and completed 3 regens in 50 miles. Has anyone with an L5P experienced any of this??

    1. I have a 2018 4×4 crew cab 3/4 ton and the same thing happened to me after they did the reflash. My DEF consumption has doubled, and my fuel mileage dropped almost 2 mpg…and I think this is due to the excess regens that started right after the reflash. I use it mostly for highway driving and tow a 13,000 lb 5th wheel on the weekends. I have really noticed the lack of power while towing. The dealer states that they can’t find anything and that this is normal. I love the truck, but it’s not the truck I bought 4 years ago!

      1. I have a 2018 as well and it has 130k on it. About two weeks i started the truck and it flashed (service emmisions system, derate in 175 miles). After a week at the dealer they determined it was the def dower nozzle while it had multiple codes for nox sensors bank one and two. After replacing the doseing nozzle it did great for about 2k miles. So i returned to the dealer and it was reading the same codes. They then told me that their recommended replacing all 8 injectors due to an injector imbalance. After getting a second opinion. It seems to be an injector pig tail on number 1.

    2. Yea in having a lot off issues with my 2020 Duramax it’s drinking fuel in death the same I mean I get I used to like five something to a tank almost 600 miles and now I get three and I’m putting in 5 gallons of death every 2 weeks not even I just put it in a week and a half ago I do have the banks I Dash and I do have the pedal monster to Pedal monster is pretty much off it’s on City turned all the way down and the banks get I think I have it programmed right but I messed with it the other day and it got even worse I mean yeah I had tons of power but the fuel consumption and Def consumption in 3 days and went to a half a tank of death and it sucked down $200 worth of fuel in no time. I have 96,000 on it and another issue I have is the front ends are f****** horrible I’ve already put everything brand new in the front end and I go through upper ball joints and control arms cuz they’re together I’ve already been through two sets and then I’ve already been through four wheel bearings on the front driver side has anybody else experiencing that

  4. Have a 2017 duramax and around 35000 miles changed fuel filter and found filings and fibre particaless in the Filter, went another 5000 miles and done another Analysis done and the numbers jumped up much higher,is any one else have this problem

  5. The 2020 L5P has plenty of torque to pull it’s rated loads. The 10L1000 Allison certified trans shifts excellent. Truck has been back to the shop only for a dealer DEF reflash – update. Unfortunately, there is the ever annoying engine thumping sound which GM fails to define what is the actual cause. Gets old real quick when all they can say is remove the fan belt and if the noise goes away it is inherent in the engine. Yes GM, but what is the cause? No one is saying!

  6. I have a 2018 3500 Denali, About six months ago it was stuck and regain 300 miles. But I was puttering around in townI normally tow toy hauler with no issues at all. I think the truck was stuck in Derated power for the first 55,000 miles after I had the DEf Flash I believe the truck has more power and using less DEF. I will know this weekend after towing If there was an improvement .

  7. My 2018 GMC Duramax with 54k on the clock drips coolant from the right rear corner of the bell housing while setting for a few days. Doesn’t seem to lose any while driving and I can’t see where it is coming from. Read somewhere that there are o rings between the engine and transmission? Going to drive it a while and see what develops. Other than that we love the truck, but don’t want to deal with problems. K&N air filter, 22 MPG @ 72 MPH.

    1. Anyone can guess where your coolant leak is coming from. May I suggest parking it on a raised platform and then crawl underneath to see if you can locate the source of the leak. Possibly a leaking heater hose seal ? I like to check simple problems first.

      1. That’s what I thought at first, but I don’t see any drips or a trail anywhere higher than some staining about half way up on a seem on the bell housing. Then the drip on the very bottom. Thanks for the input.

      2. Anthony, dealership found a crack in the rear engine cover, it’s inside right below the rectangular port guys talk about leaking. I have pictures if you’re interested. Thanks

  8. Hi all, I have a 2019 2500 L5P, 7in lift and 37in tires, Banks Derringer, pedal monster, Banks air intake and Idash. At 30,000 it would go into limp mode while driving down the highway and I would pull over turn it off and back on and it would go fine. After the 2nd episode I took it to my dealer (had a 3rd episode on the way there). They had it for 2 weeks and replaced the fuel pump module. On my drive home it did it again. I took it back and they replaced the #4 injector connector and it has been fine since, now has 45,000 on it. For the last 20,000 miles I have been running a Banks derringer and Pedal monster and for you guys lacking power, those 2 items are amazing. These items are a game changer while hauling my toy hauler through Arizona mountains. I also have the Banks Idash and have watched my regen cycles and I have noticed when I run the truck in “stock” mode it seems to regen more than when I have the derringer on level 3 and the pedal monster on street driving. I believe the added acceleration power helps me keep my foot out of it more, I’m not sure but that is what I noticed.

  9. Got a check engine light after 84000 miles. I have had really 0 problems to date but here are a couple of things I have noticed in the 4 years I have had my 2018 3500. One, I got the regen keep driving notification one time about 15000 miles. Never saw it again. The consumption of DEF has gone way down from when I first started putting it in the truck. Thought this was normal as there are no guidelines as to how much you should use. Do I have a problem or am I just lucky? Don’t fix it if it ain’t broke right? Seems like the likely culprit for check engine light is going to be the MAP sensor. Seems to be running fine, but the lie-o-meter is indicating lower than normal fuel miles per gallon. Any thoughts? Should i replace the MaP myself or let dealer do with extended warranty I haven’t used yet?

  10. you may encounter connector problems on more than just fuel injector #4. Mine happened on #6 and one guy on youtube has replaced #2, #4, #5 and #7 plus #7 injector. I’ve heard of #3 going too so not just limited to #4. They should recall and replace the harnesses on both sides. This is starting to become more common. Left me stranded in the middle of nowhere.

    1. Do you know cost to replace per connector@ dealership?
      Researching gas vs diesel.
      I can do MAP clean and DEF reloads myself no issue there. Comparing L5P to L8T so far diesel wins. It seems L8T has more issues and that oil burn.
      Looking at your experience , Do you think diesel would do well in 40city/60hwy driving avg.15k miles / year?
      It seems diesel is better build and less maintenance.

  11. I have a 2017 3500HD w Duramax L5P bought it used with 126K miles and now has 233K. My problem is the soot levels are building and it won’t REGEN because the and NOX and EGR sensors are bad. There is currently a nationwide backorder on the NOX2 sensor, been that way for the past 4 months with no estimated delivery date. I tried 4 dealers, 2 parts stores and 2 diesel specialist shops with no luck. Any thoughts on how to source the parts from Canada or elsewhere? Or other suggestions?

  12. We have a 2017 Denali 3500. Continuous nox sensor replacements and maf trouble. (Spacer installed). Had to replace front hubs, tierods pitman and idler, trans cooler line. Now coolant level sensor. Got a new tank to go in. Yes filled the bottom of reservoir. Drivers window when rolled up rolles back down unless you hold the switch up. Now wipers make a few swipes when starting on a random basis.. this truck has 117000 miles 0n it. They done make them like they used too.

    1. your truck must have been right behind mine in the manufacturing line… i’ve got 185k and ALL the same repairs/issues. Now I’m having issues with radiator with caused the motor to overheat and warp heads. Im on the fence of having heads machined (so i know what i have and what i am getting) versus replacing the motor which potentially could be a lemon in a box.

  13. 2019 GMC 3500 Denali – 121k miles.

    P0191 and 1050. Power reduction and Def Level sensor out. I truly hate this truck. Will be going back to stealership for the 4th time.

  14. 2019 gmc 2500 Denali -48000 miles

    This truck has been to dealer for coolant tank failure at 20k miles, crank balance pulley failure at 35k miles, and now drive idle pulley failure at 47k miles. Never threw a code for any of the failures. Thankfully still under warranty but with this many problems it makes me nervous with what is to come. Truck has had regular service at the severe use intervals since I tow a 10000lb trailer frequently. All hwy miles and now transmission is starting to act up when accelerating from a stop or when rapidly decelerating. Back to the dealer we go.

  15. I like to check the MPG on my 2022 AT4 2500 occasionally to see what I’m getting using the DIC. There have been a couple times that I have checked and the display says 11 MPG where I normally see 19-22 MPG. This has happened 3 or 4 times and then the MPG mysteriously returns to normal. Any ideas? Thanks.

  16. I have a 2017 Duramax 2500 that has a code P2294 won’t start up now Any suggestions why. Ran fine till I shut it off

    1. Lewis – that code is for the fuel pressure regulator. Have you tested fueling/fuel pressure to ensure the engine is receiving fuel?

  17. I posted back in June about my 2018 GMC (61,000) with a coolant drip off the right bottom of the torque converter housing. As bad as I hate to take my truck to a dealership, I broke down and did. I was told it would be a 2-3 day job, told them no hurry I want it done right. Day 4, truck might not be done by day 5 and could possibly be Monday. Monday I learn their waiting on a gasket, time to visit the dealership, SW brings out the rear engine cover that has a crack inside below the rectangular port guys talk about, and the replacement was found to be leaking in the vicinity of the rear seal. Suspicious that a new casting is cracked, not confident in this dealership mechanic at this point. 5000 mile trip coming up, wish me luck. I can send pictures if anyone wants.

      1. Shop had hours stopping coolant leak near upper bell housing. Difficult to locate let alone repair.
        Now cracks are cause.
        Labor intensive job

  18. I have a 2017 3500HD L5p
    I had #4 injector failed and it over fueled the cylinder causing fuel to mix with the oil and know I replaced the #4 injector and fuel rail and lines,sensor’s and it will fire up and run but it sounds like a horable rod knock I towed it to the shop to see if I missed something and the shop is telling me I need a motor and quoting me $23k there telling me I have a dead cylinder because of the #4 injector!!! The truck only has 134,000 miles has anyone else had this happen or knows what I can do to get it fixed is there some kind of recall though the dealership or something I just bought this truck about 5months ago

    1. They love to pull the new engine deal don’t they. Dodge In Vernon BC did that to me until I had it towed 700km to my guy and ended up being a dead oll pressure gauge. Vernon Dodge – Lying scam artists.

      I would assume at worst you would need a new injector, bearings, possibly ( doubtful but possible) a new rod and wrist pin assembly. If you can get the oil pan off, have a look at the bearings because that’s what will take the beating .first when you hydraulic If the bearings are toast you could still be all right because as long as you don’t have to pull that rod and piston, it’s fairly easy to address. If you do have to pull the piston, it gets bit more tricky reinstalling because can’t fit a ring compressor up there. Its not impossible. And you don’t need to buy a brans new engine off those crooks, There are deals out there, I just picked up 2021 Dmax with trans and computers plus all the wires, 14000 km on it all for 15,000.00 Canadian it’s going into my 2017. So I’ll have a 2017 L5p available shortly . High miles but good long block for you to swap over to.

  19. Shop had hours stopping coolant leak near upper bell housing. Difficult to locate let alone repair.
    Was told waiting for gasket also.

    Now others are having cracks, are cause.
    Labor intensive job, without cracked

  20. Good day all,
    I have a 2017 High Country D’max / Alli and I picked it up from a dealer in Edmonton with 750,000 km’s on it. Came with 390 Liter slip tank with toolbox ( 3000.00) 26k Curt 5th wheel ( 2500.00) Gooseneck ( 300.00) They did just over 7000.00 to steering, brakes, suspension, tires. I paid 30,ooo.00 CDN. It started going into limp mode once in a while, bought a programmer. Limp mode got worse and worse until I parked it. Initially i had a 2020 drivetrain lined up but the guy sold it under me., Now, I just bought 2021 D/max Denali with the Alli, that was written off from a tree falling across the box. I have all the computers, Key Fobs, engine wiring, main fuse box, dash cluster, center controls for stereo, climate etc. Steering column and wheel, All the dash wiring, complete dash. Intercooler, trans cooler, rad, battery wiring, engine mounts, basically everything the guy had left. Now..I’m getting told 50/50 on the swap working. What issues am i going to run into. Oh yeah, its all got 14,000km on it and I paid 15,000.00 CDN for everything. The truck is in mint condition and if i covered the mileage anyone would guess well under 50,000km it’s that clean and very well looked after. I figure if the swap works seamlessly, truck should be valued around 60 – 75k CDN if this works out and I’ll end up with a good truck that will last me until I retire in 5 years. Any comments, suggestions, experiences with this type of swap would be greatly appreciated. ,


  21. I have a 2022 Chevy Silverado 2500 Duramax. I bought this thing new. At 8000 miles it got a fluid leak between the transmission and transfer case. I took it in and they replaced the seals. Then with in 150 miles it came back. I took it back in they replaced the transmission. I drove it about 10 miles and it started leaking again. Took it back in they replaced the transfer case. Problem fix but around 16000 miles it started leaking again this time from the bell housing. I took it in they told me it was the stator leaking and it was unavailable nationwide. This was ordered 3/27/23. Just got a call from they saying the part would be in 5/28/23. All right that’s great. So I changed the fuel filter on this thing and found metal or aluminum shaving in the filter. Boy I can’t win for nothing.

  22. I have a 2018 L5P, with 144, 000 miles. The number 4 pigtail went out when the truck had approximately 100K in mileage. Long ways from home so took it to the nearest dealership. They had the truck for a week, but that was trying to get the part. Service guy said they have had many of them go bad. This week, got a PO205 code saying number 5 injector misfired. Limped it home due to work and then called the dealership to have it looked at. Not sure what level of inspection they did, but they are saying new injector, fuel line to the injector is one time use only (their words) and new pigtail. Also told me I need to change my air filter even though I had done it two weeks prior. Beginning to question their level of expertise and honesty. I previously had a 2005 with a duramax and sold it with 335, 000 miles and never did any engine work. This new one is not making me all warm and fuzzy inside.

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