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 He has been the lead writer for Chevy Trucks and has transformed it from the old and outdated site it was into what it is today.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.

Following the short lived LMM engine, the Duramax LML was introduced in 2011. Featured in the same 2500 and 3500 HD work trucks, the LML sported the same 6.6L turbo-diesel design that Chevy diesel trucks have been using since 2001. However, the LML was actually the biggest overhaul Duramax had done to the 6.6, claiming to use nearly 60% of freshly designed parts not used in previous version.

Frequent across all diesel engine manufacturers, the engine update was primarily driven by increasing diesel emission standards. While the prior LMM had added DPF, or diesel particulate filters, the LML had two new, major emissions upgrades with the addition of selective catalytic reduction (SCR) and diesel exhaust fluid (DEF). Due to these advanced emissions systems, the LML had the longest production history of the 6.6’s to-date, lasting until 2016 after which it was replaced with the L5P.

At 400hp and 765tq, the LML is also the highest power producing Duramax made to-date, up until the L5P.

Duramax LML vs. LMM Improvements

Because the LML had some major changes, I want to briefly cover its differences compared to the LMM.

Block, Rods, & Pistons

First, the engine received a strengthened block and improved main bearing to improve on previous weaknesses. Additionally, the rods and pistons were both strengthened while actually being lighter, creating a lighter rotating assembly. While the rods and pistons are both made similarly and out of the same material as the LBZ/LMM rods and pistons, the piston wrist pins had modified ends for more support and the rods featured a cracked cap design. All of these changes significantly improved the durability and strength of the rods and pistons over the previous versions.

With the improved rods and pistons and lighter rotating assembly, cracking is no longer a common issue as it was on LMM and LBZ models and the internals can now handle up to approx 700rwhp without any issues.

Injectors & Injection Pump

The LML included an advanced injector design, capable of 30,000psi of pressure compared to 26,000 on the LMM. The increased pressure capacity allows the engine to produce the significant power.

To support the higher pressure injectors, the LML was fitted with the Bosch CP4.2, upgraded from the previous Bosch CP3. Despite the upgraded injection pump, it actually flowed 20% less fuel than the CP3, creating problems with performance modification and tuning. Additionally, the injection pump does not take well to contaminates and poor lubrication, causing a myriad of potential problems which we’ll cover later on.

DPF & a 9th Injector

The DPF regeneration process was overhauled by adding a 9th injector in front of the diesel oxidation cat which is located within the turbo downpipe. The 9th injector receives fuel from the CP4.2 and injects it directly into the exhaust system. This removed the oil dilution issues with the LMM engine and increased the regeneration intervals.


Selective catalytic reduction was added to the engine, which injects a special fluid, known as diesel exhaust fluid into the exhaust system. The exhaust fluid creates a chemical reaction which convers nitrogen oxide (the bad stuff) into nitrogen, water, and carbon dioxide. While these systems further complicate the broader emissions process, they actually improved fuel economy by about 10%.

The drawback is that you now have to also fill your Duramax engine up with diesel exhaust fluid on a regular basis, although it is relatively inexpensive and offset by the fuel economy gains.

The 4 Most Common 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:

Duramax LML CP4.2 Injection Pump Failure

Duramax “upgraded” the injection pump for the LML models, going from the Bosch CP3 to the CP4.2. To handle the additional power capabilities of the engine the fuel rail pressure was increased to 30,000psi. However, the pump actually flowed 20% less fuel than its CP3 predecessor. 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. The fuel pump has two piston-like components inside of it and it only uses diesel fuel for lubrication. Diesel fuel is highly susceptible to picking up water, dirt, and other contaminates. Because neither water nor dirt are good lubricants, these getting into the fuel pump can wear down the internal components and cause disastrous damages. Once one small shaving starts circulating, a chain reaction begins and the CP4.2 ends up destroying itself from the inside, while taking the whole fuel system out with it.

Small aluminum bits can shave off of the internals of the pump and then get circulated through the whole fuel delivery system. The fuel rail, the injectors, fuel lines, etc. all will have metal shavings circulating throughout. Once this happens, the fuel pump will fully die, leaving your engine starving of fuel.

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

As you can imagine, this is a very expensive job. A failed CP4.2 injection pump in an LML can be a $10,000+ repair job for all parts and labor. Because the primary cause of failure is bad diesel fuel, this can happen on an LML with 10,000 or 200,000 miles. Additionally because of this, adding a lift pump won’t solve the problem either (although it will solve the added power problem).

Most owners consider the injection pump a ticking time bomb and the best “replacement” option is just to swap it out for a different injection pump before it breaks.

Diesel Fuel Quality

Always use the best diesel fuel. Even if you don’t have the CP4.2 injection pump on your truck anymore, diesel fuel is still extremely important. Bad fuel will jack up your injectors and cause your engine to run poorly.

Use a high quality fuel additive to help treat diesel fuel – we recommend Stanadyne.

Try to fill up on fuel from gas stations that get a lot of traffic and sell a lot of fuel. Diesel fuel sitting in tanks for months because it never gets bought is an easy way for it to accumulate excess dirt and contaminates.

Also, replace your fuel filters frequently.

2. DEF Heater Problems

One of the emissions additions to the LML was the inclusion of a selective catalytic reduction (or SCR) system as mentioned at the start of this article. SCR requires diesel exhaust fluid “DEF” to operate properly. Therefore, the LML has a separate DEF fluid tank in additional to the normal diesel fuel tank.

DEF fluid consists of about 67.5% water, with the remaining being urea. Due to the high water content, DEF fluid is prone to freezing. Frozen DEF doesn’t quite work very well so the system is accompanied by three heaters. One in the fluid tank, one in the DEF pump, and one in the supply line which delivers the fluid to the injector.

When DEF fluid gets to a certain temperature, the temperature sensor alerts the glow plug control module (GPCM) to kick the heaters on.

The DEF heaters 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. Alternatively, completely removing the SCR & DEF systems is somewhat common, although illegal per emissions laws.

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.

Poor DEF Quality Fault Warning

The engine computer takes the reading from the first NOx sensor and compares it to the reading on the second sensor. When the amounts of NOx have not been adequately reduced from sensor 1 to sensor 2, the system assumes there is an issue with the DEF. When the NOx sensors lose calibration, the NOx readings are not properly relayed to the engine’s computer.

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.

You will need to take your LML to a Chevy dealer or to someone with the proper software to correct the issue. The sensors will need to be recalibrated to the engine’s computer which requires specific GM/Chevy diagnostic equipment.

4. DEF Pump Failure

If you’ve noticed a trend with the LML, it’s that the DEF systems are problematic. This was new technology at the time and was bound to have its fair share of issues. On the LML, the DEF system is equipped with a typical pump. The pump sends the DEF fluid from the reservoir tank to the SCR system located in the exhaust.

As with any fluid pump, it’s prone to failure via normal wear and tear, dirt and contaminates in the fluid, etc. Failure on the LML’s hasn’t exactly been pinpointed but the pumps have been known to fail as early as 30,000 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.

LML DEF Pump Failure Symptoms

  • DEF light illuminates
  • Bad DEF quality message

The replacement options here are either to delete the DEF system or replace the pump. The pump is located within the reservoir tank.

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. Various lawsuits have been filed here and there might be a way to receive some type of compensation, but for the most part this is your bill to bite.

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. DEF and SCR cause their fair share of problems, but are also covered under warranty for the trucks that still have it. The problems are common enough that deleting the DEF system is frequent among LML owners. But do note this is technically illegal.

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.

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