Duramax LML Engine Problems

The 4 Most Common Duramax LML 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 He has been the lead writer for Chevy Trucks and has transformed it from the old and outdated site it was into what it is today.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 LML was the biggest overhaul Duramax had done to the 6.6L diesel, using 60% new parts. It received major changes to the block, rod, pistons, emissions equipment and fueling system.

The big engine changes to the block and internal components led to a stronger and more durable engine, but the new fueling system and emissions equipment plagued the LML with problems. While I still consider the LML to be a fairly reliable engine it did have some expensive problems like injection pump failure and DEF issues.

The LML engine can make it to 300,000 to 400,000 miles reliably, you just have to be careful about the injection pump and emissions systems.

Duramax LML Engine Problems

  • CP4.2 Injection Pump Failure
  • DEF Heater Problems
  • NOx Sensor Failure
  • DEF Pump Failure

The buzzkill of the LML is the CP4.2 injection pump. Outside of this problem, there aren’t many major or costly common problems associated with the engine. Fortunately, there are some relatively simple ways to prevent these issues and make them less common.

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

1) CP4.2 Injection Pump Failure

Duramax “upgraded” the injection pump for the LML models, going from the Bosch CP3 to the CP4.2. The new CP4.2 pump actually flowed 20% less fuel than the CP3. This makes the CP4.2 highly incapable of handling additional horsepower without the addition of a lift pump.

But, added power isn’t the achilles heel of the CP4.2, bad fuel is. Dirty fuel wears down on the internal components which causes metal shavings to start circulating throughout the fueling system, causing the CP4.2 to self-destruct, taking the whole fueling system out with it.

The fuel rail, the injectors, fuel lines, etc. all will have metal shavings circulating throughout, causing injectors to fail and the fuel lines to need to be completely flushed or replaced too.

The injection pumps on the LML usually fail with little to no warning, and total failure only takes a few minutes once the first symptom occurs.

Injection Pump Failure Symptoms

  • Poor performance and idling
  • Cylinder misfires
  • P0087, P0088, P0191 or P128E engine codes

Replacement Options

When the LML’s CP4.2 injection pump fails, Chevrolet recommends replacing:

  • Injection pump
  • Fuel rails
  • Fuel injectors
  • High-pressure fuel delivery lines
  • Low-pressure return lines
  • Clean out fuel tank, sending unit, and all main lines

All-in-all you end up with a $10,000+ repair bill. The CP4.2 is a ticking time bomb and the best “replacement” option is just to swap it out for a different injection pump before it breaks.

2. DEF Heater Problems

The addition of selective catalytic reduction (SCR) to the LML required the use of DEF fluid. DEF fluid is prone to freezing because it has a high water content, so a heater was added to the system to prevent freezing.

The DEF heaters on the LML is known to fail frequently, sending the engine into limp model with reduced power.

Engine DTC Codes for Failed DEF Heater

  • P21DD
  • P20B9
  • Limp mode (will appear on heads up display)

Replacement Options

There are three heaters located through the DEF system and all are prone to failure. The only replacement option is to replace the heater, unless you want to bypass the code and limp mode. For some individuals who live in very warm climates where the temperature never drops below freezing, the DEF heater is pretty irrelevant.

However, whether you need the heater or not, a failed heater will throw codes and put the car in limp mode. If you don’t want to spend the $300-$500 replacing the heater there are a number of ways to bypass the code.

3. NOx Sensor Failure

The goal of DEF and the SCR system is to reduce the amount of nitrogen oxide that enters the atmosphere via exhaust gases. The system has two NOx sensors: one before the DEF injection location and one after. The goal of the NOx sensor is to ensure that the fluid is adequately reducing nitrogen oxide levels.

Specifically in 2011 LML’s, the NOx sensors had a design fault which caused them to either fail or lose calibration with the ECM.

The result of a faulty sensor is receiving a “Poor DEF Quality” message. The message will give you a certain amount of miles to resolve the code otherwise it will send your engine into limp mode. Other common NOx sensor symptoms are these engine codes:

  • P20EE
  • P207F

The sensors do not measure DEF quality! Therefore if you receive this code, simply replacing your DEF fluid will not cut it. The sensors will need to be recalibrated to the engine’s computer which requires specific GM/Chevy diagnostic equipment.

4. DEF Pump Failure

The DEF system uses a pump that sends fluid from the tank to the SCR system. These pumps commonly fail every 30,000-50,000 miles.

When the pump fails, no fluid gets sent to the SCR system. Therefore no NOx reduction takes place, causing the NOx sensors to give readings in the same range. This causes the “bad DEF quality” light to come on which will again give you the mileage countdown until limp mode.


  • DEF light illuminates
  • Bad DEF quality message

Duramax LML Reliability

The biggest concern on any LML is the CP4 internally combusting. The associated cost of this is about $10k and it’s not covered under warranty because it is caused by poor fuel.

However, the CP4.2 failing is usually debated to be “not as common” as people make it out to seem. The commonality of this has certainly been somewhat inflated due to the cost of the repair. But, any problem that can occur out of nowhere that costs $10k is going to get a lot of attention. The best peace-of-mind option here is to do a CP3 conversion.

Outside of the injection pump, the LML is a reliable engine with the exception of the emissions systems. Beyond emissions systems and the injection pump, the LML is the strongest Duramax engine built to date. The block, rods, and pistons on prior Duramax engines were common failure points on tuned and modified engines due to weak components. All of these items received upgrades in the LML. With proper preventative maintenance, the LML engine itself is easily capable of 300,000 to 400,000 miles.

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  1. One would think that if the CP4 pump is a design flaw and intended for the thicker European fuel GM would change out the pump for the CP3 pump at no cost or no more than the cost of labor, they say the fuel used is bad . That’s like saying the Refinery is producing shoddy fuel, maybe we should blame them , yeah right, see where that gets you. I use Lucas upper cylinder lubricant about 10, ounces per 20 gallons of fuel. I never let my tank get below 1/2 tank , I use fuel injection cleaner every 3,000 miles , and change out the fuel filter every other oil change, I’m doing the best I can in light of a potential catastrophe of the CP4 pump so far so good, I did have my DEF tank changed out due to a heater problem, warranty covered that ,
    My truck is a 2015 LTZ 4×4 crew 3500HD sew
    6.6 duramax with just under 60,000 miles it has been very reliable, just a little uneasy about the ticking time bomb CP4 pump

    1. I have a 2016 Chevy LML Duramax. Installed a lift pump after warranty expired. I Change the Fuel Filters and Water Separator every time the truck gets to 50% fuel filter life. As of to date I’m at 227,ooo miles and going strong. Yes, I feel extremely lucky..!!

  2. How can a CP 3 pump work in a LML ?
    It was designed for 26,000 pal, not 30,000 like the CP 4. Thx

      1. I did the cp3 swap on my 2011 Duramax and haven’t had any issues. You will need it re tuned for the new fuel curve but that was no big deal. But I did notice about 5 mpg less than what I did average with the cp4 but I also have it on a 200hp tune

  3. I haul a loaded horse trailer across some hot deserts in the summertime. My 2016 duramax lml went into limp mode due to a bad DEF heater on a trip which left me stranded. Both the trailer and truck require towing putting my horses at risk due to summer temperatures. I need a way to bypass DEF issues until I can safely make it to a dealer otherwise I am looking for a gas truck. Any suggestions? Thanks in advance.
    Also interested in the CP3 conversion!

    1. Depending on the state you live in, you can do an EGR and DEF delete which totally does away with DEF and the exhaust gas resirc. which will not only make your truck run better but it also makes the hassle of DEF go away. This will cost around a couple thousand if you do it the right way. I have a 2016 and I have it fully deleted with EFI live 5 switch tunes and my truck runs like a dream and its pretty quick on top of that. Although, they recommend having a lift pump if you use the 5th tune.

  4. A popular online autoparts retailer sells the new Bosch and GM fuel injection pumps. I’m assuming the GM pumps are made by Bosch. If I go with their new pump either GM or Bosch (Bosch is sold out at the moment), I wonder if I am getting an upgraded pump? ie have the new CP4.2 pumps been upgraded? I am currently running the original CP4 pump. I’d prefer buying a new pump as compared to a remanufactured pump. I believe my local GM shop foreman told me they upgraded the LML pumps in 2016??? I know that in 2017 GM went to a Denso injection pump and a turbine tank pump. In 2011 the tank pump was a jet pump.

    1. Norm – P0088 is fuel rail pressure too high. Are you tuned or running any mods? If not, we’ve seen this happen before from a bad battery. Alternator overcharges the battery above 15 volts which increases fuel pressure. Alternatively it could be a fuel pump issue.

  5. Hi, I have a 2016 Duramax. I tow a 26ft enclosed trailer with a RZR in it. My mileage towing is about 8 mpg. I drive slowly usually under 65 mph. I bought a diesel hoping for better mileage, very disappointed. Not only does the fuel cost more. Oil changes, fuel treatment, DEF, and fuel filters are more. I am currently getting a code P205B, reductant tank temperature sensor performance. Any advice on this over priced truck?
    Thank You!

    1. Hey Jeff – that engine code is usually caused by a bad DEF pump and actually results in a pretty significant decrease in MPG so guessing that is the cause for you fuel economy issues. Usually requires the whole tank assembly/module to be replaced. Also, tuner and an intake can help you get probably 3-4mpg better.

    2. Hi Jeff,
      I have a 2015 3500 Duelly and tow a 5th wheel toy hauler that weighs 19k pounds. I also get 8 MPG towing and 17 MPG not towing. Before I purchased people said you get the same fuel mileage empty or loaded, BULL…. Mine is stock. and have had the DEF heater replaced three times and the entire tank assembly replaced once, mileage has always been the same. Going to try the intake and see if fuel mileage increases, but with the price of “upgrades” to improve fuel mileage it’s almost not worth the cost.

  6. Hey Jake, I have a 2015 Chevy 3500HD Duramax that has been in the shop going on 8 weeks now and they can’t figure how to get it out of limp mode. Readers Digest version, display says DEF quality poor, 96 miles until you are reduced to 65mph. On Jan 1st DEF light and DTC (service engine soon) lights illuminated w/ the following codes (P249D- closed loop reductant injection control at limit- flow to low) & (P22FE) NOx Sensor 2 performance- sensing element). Replaced the DEF injector and cleared code, had to take it to the dealer for the NOx sensor calibration. Took them almost 2 weeks to diagnose and they told me NOx sensor #1 was bad, i told them it threw NOx sensor code #2 not #1. They changed it and tried to run a quality test, which did not pass on multiple tries. 2 weeks later they told me NOx sensor #3 was bad, replaced that and tried to run quality tests again without success. At week 4 they said the SCR/DPF is bad and GM has zero in stock. I found a company DPF360 (I believe they are out of Sumerville, AL) who specializes in reconditioned OEM SCR/DPF parts. Ordered it (for $1224.00 vice the $3k the dealership wanted) and had it installed by a custom exhaust shop as the dealership where my truck is could not accomplish that. After getting the truck back and the mechanic driving it and doing multiple re-gens they still can’t get it to pass the quality test. I asked them if the firmware is up to date, they made sure it was.
    I am at my wits end as GM cannot figure out WHY and they are unable to fix my truck at this point.



  7. I have a 2014 LML with 563, 866 miles, i had to replace the CP4 at 498k (Caught it before metal went through the system) I added a lift pump at around 350k and i use Lucus fuel injector cleaner/lubricant with every tank of fuel. Still runs great.

  8. One of the CP3 replacement kit manufacturer’s engineering leads shared some detail around the CP4 failure issue with me earlier this year. From what they have been able to discern, it’s the sleeve bushing bearings on the eccentric follower roller(s) in the CP4, similar to roller lifters on roller cam setups. These drive the pump, and wear due to the significant mechanical stresses on the parts. The worn bushing eventually causes a roller to cock to one side. when this happens, it galls and fractures itself and the cam, and very quickly causes massive failure due to significant metal contamination. From their analysis, it’s going to happen regardless of ANY level of fuel contamination. So that sounds like it’s a design issue, not fuel contamination.

  9. We have a 2012 Chevy 3500 in service as an ambulance chassis with 75K miles. It started to repeatedly display the exhaust warning light and no amount of driving would keep it out. We had the filter cleaned and are still repeatedly having problems, including it going in to limp mode. The report from the filter cleaning showed by weight it coming out all but in new condition. What else can be causing this and is there a way to bypass the limp mode.

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