The 2011–2016 LML Duramax 6.6L V8 diesel engine, found in GM’s heavy-duty Silverado and Sierra HD trucks, is known for its towing power, emissions advancements, and smooth performance. Introduced as an upgrade over the previous LMM, the LML brought increased horsepower and torque, plus the integration of newer emissions technology like DEF injection and an SCR system.

But like any engine platform, the LML has its strengths and weaknesses. This post covers what you need to know about its reliability and common issues, based on GM technical service bulletins (TSBs) and feedback from owners and forums.

What the LML Duramax Does Well

Overall Reliability

When properly maintained, the LML is a solid and dependable platform. Many owners report their engines running well past 200,000 miles with regular service and proper fuel quality. Compared to some earlier models, the LML benefits from improved piston design, stronger internals, and an updated CP4 injection pump (though this has its own caveats, more below).

Improved Power and Emissions Compliance

The LML made 397 hp and 765 lb-ft of torque from the factory, more than enough for towing and hauling. GM also added a DEF system to meet 2010 emissions standards, helping the engine burn cleaner and keep EGR soot out of the intake system (at least to a degree).

Quieter and Smoother Operation

Thanks to the high-pressure Piezo injectors and refined tuning, the LML is noticeably quieter and more responsive compared to older Duramax generations like the LB7 and LLY.

Common Issues with the 2011–2016 LML Duramax

CP4.2 Injection Pump Failure

• Severity: HIGH CONCERN

• GM TSBs & Field Reports: Numerous GM service bulletins have noted issues with premature CP4.2 pump failure.

• Owner Feedback: This is the single most notorious issue on the LML platform. If the CP4 injection pump fails, it can send metal shavings through the entire fuel system leading to a $8,000–$12,000 repair bill. Many owners opt to install a CP3 conversion kit along with an Airdog Lift Pump to eliminate this risk. If funds are a bit tight, but you would like to band aide the issue, run the Exergy Fuel Pressure Regulator. 

DEF System and Emissions-Related Faults

• Severity: MODERATE CONCERN

• Common Codes: P20B9, P204F, P207F, Urea quality issues.

• Dealer Observations: TSBs address sensor calibration problems, frozen DEF fluid in cold climates, and contamination in DEF tanks.

• Owner Feedback: DEF tank heaters, level sensors, and SCR components have been known to fail, especially in northern climates. Many report “Service Emissions System” warnings, even when no apparent problem exists.

Glow Plug and Controller Failures

• Severity: MODERATE CONCERN

• GM TSB: #PIP5217 addresses glow plug system issues.

• Customer Complaints: Owners in colder regions report failed glow plugs or control modules, leading to long crank times or cold start problems. Replacement is relatively simple, but it's a common issue in higher-mileage LMLs.

Sticking Turbo Vanes (Sensor or Turbo issue)

• Severity: LOW CONCERN

• Symptoms: Reduced power, check engine light, turbo performance issues.

• Owner Observations: Many LML owners have dealt with turbo vanes sticking due to soot buildup, or sensor failure. This can usually be corrected by cleaning or replacing the vane position sensor. In some high mileage cases the build up may be too much and turbo replacement could be necessary. It is best to have your truck diagnosed before throwing parts at it. The technician will manually sweep the vanes to see if the turbo tends to stick in a certain location. 

Broken Crankshafts

• Severity: LOW CONCERN

• Symptoms: Reduced power, rattling in engine bay, check engine light

• Owner Observations: Some higher mileage LML owners have run into issues with the Crankshafts shearing near from the front. The firing order of the LML creates a high stress point on the front of the crank, this can be relieved by installing an Alternate Firing Order Camshaft. Note, if the truck already has mid to higher mileage, you should replace the Crankshaft while doing this Camshaft. 

LML Maintenance Tips for Longevity

• Use high-quality fuel and additives to reduce wear on the CP4 pump. When ready, replace the CP4 with a CP3 so the failure is a worry of the past.

• Install a lift pump (like the Fass or AirDog) to reduce strain on the injection system.

• Run the truck a bit harder than usual every once in a while, this will heat up the DEF/Regen system to help clear it.
  

• Stick to OEM or trusted aftermarket filters, especially fuel filters, every other engine oil change.

Final Thoughts

The 2011–2016 LML Duramax is widely appreciated for its power, towing capability, and relative refinement. However, the CP4 pump and emissions-related gremlins are well-documented concerns that any owner—or buyer—should be aware of.

If you’re proactive with maintenance and preventive upgrades, an LML can deliver hundreds of thousands of miles of dependable service. For many diesel enthusiasts, especially those towing regularly, it strikes a good balance of performance and modern features, just be prepared for some added complexity compared to earlier Duramax engines.

Have an LML Duramax? Let us know in the comments what issues you've faced—or how many trouble-free miles you’ve racked up!