What the LML got right
The 2011 LML was rated at 397 hp and 765 lb-ft of torque, a big step over the LMM it replaced. GM also fixed a problem that hurt the LMM. On the LMM, regeneration fuel was injected late in the engine's cycle, and that fuel could end up in the engine oil. The LML moved that job to a ninth injector in the turbo downpipe. Fuel goes into the exhaust, not the cylinders, so the LML does not have the oil dilution problem that came with the LMM.
The rotating assembly also improved. The LML pistons have no wrist pin bushings, which were a weak point on the LBZ and LMM, and the connecting rods are lighter. The engine's problems are mostly in the fuel system and the emissions hardware around it, not the bottom end.
Problem 1: CP4.2 high-pressure fuel pump failure
This is the main one.
What it is. The LML uses a Bosch CP4.2 high-pressure pump. Diesel fuel is the only lubricant inside it. Dirt, water, air, or poor lubricity can wear the pump, and the metal debris that comes off the pump travels into the high-pressure side of the fuel system.
Why fuel quality matters. Fuel has to lubricate the pump's moving parts, and the lubricity limit is looser in the United States than in Europe. ASTM D975, the U.S. diesel standard, allows a wear scar up to 520 microns in the HFRR lubricity test. Europe's EN 590 limit is 460 microns, and the Engine Manufacturers Association recommends diesel at 460 microns or below to protect injection systems. Fuel that only just meets the U.S. limit gives the pump less margin. We did not find a Bosch document that states a pump-specific number, so treat this as the standards gap, not a Bosch specification.
Why the LML is exposed. GM did not put an electric lift pump on the LML. The CP4.2 has to pull fuel from the tank itself, and it does not tolerate air or low fuel pressure well. Ford's 6.7L Power Stroke uses a very similar pump but comes from the factory with an electric lift pump feeding it, which is a big reason it has held up better.
Symptoms and codes. GM Service Bulletin 16-NA-102 covers 2010 to 2016 Silverado, Sierra, Express, and Savana diesels with LML or LGH engines. It lists the complaint as hard start or no start, with codes P0087, P0088, P0191, or P128E. The bulletin warns that normal diagnostics can be inconclusive or lead straight to a pump replacement.
What the repair involves. If the bulletin's inspection finds magnetic metal debris, GM's repair is not a pump swap. The procedure calls for:
- A new fuel pump kit, which GM lists as containing the pump and injectors
- New high-pressure fuel pipes, fuel hoses, and the indirect (ninth) injector
- Cleaning and flushing the chassis fuel lines, filter pipes, and engine-mounted fuel lines
- A cleaned fuel filter housing with a new filter
- A cleaned and flushed fuel tank and fuel sender
GM's own bulletin lists the labor for this job at 18 to 21 hours depending on model year and drivetrain. That is why a CP4.2 failure is a repair bill you want to avoid, not just a part to replace.
The bulletin's labor times by model year and drivetrain are 18 hours (2011 to 2014 2WD), 20 hours (2011 to 2014 4WD), and 21 hours (2015 to 2016, 2WD and 4WD).
Warranty check. The bulletin notes that repairs covered under powertrain warranty use a dedicated labor operation. If your truck is still covered, or you are buying one with remaining coverage, confirm what applies to your VIN with a GM dealer before paying out of pocket.
The class action settlement is closed. Chapman v. General Motors (E.D. Michigan, No. 2:19-cv-12333) covered 2011 to 2016 Silverado and Sierra trucks with the 6.6L Duramax bought from a GM dealer in California, Florida, Illinois, Iowa, New York, Pennsylvania, or Texas between March 1, 2010 and September 13, 2024. The court gave final approval on May 6, 2025, for a $50 million settlement, and GM denied wrongdoing. Claims for past repairs and for former owners closed on November 6, 2025. A 50% reimbursement program for repairs at GM dealers ran until May 6, 2026, for trucks under 200,000 miles. Private sales and independent lot purchases were outside the class. If you are buying a used LML today, plan as though a CP4.2 repair is your cost, and check with a GM dealer or the settlement administrator if you think a past repair on your truck qualifies.
What owners do about it:
- Change the fuel filter on a schedule. Do not wait for the dash reminder. Clean fuel is the first line of defense.
- Add an aftermarket lift pump system. Drivingline calls a lift pump one of the first upgrades for any LML, especially if you plan to tune it. Ask your installer about filtration and air separation too.
- Convert to a CP3. The CP3 was used on earlier Duramax engines. Conversion kits remove the CP4.2 from the equation. Ask the kit maker whether your setup needs tuning.
If you are shopping, ask for service records on the fuel filter and any lift pump or CP3 work. A truck with a documented fuel system history is a better bet than one with none.
Problem 2: Ninth injector codes and regen faults
The ninth injector is the fix for oil dilution, but it is also a service item. GM bulletin MC-10112228-9999 covers 2011 to 2015 LML trucks with codes P0420, P2463, or P24A0 and points to the ninth injector (the hydrocarbon injector) as part of the diagnosis.
What this means for you:
- If you see those codes, have the ninth injector and its fuel supply checked before you replace expensive exhaust parts.
- The ninth injector gets its fuel from the CP4.2 side of the system. GM's CP4.2 bulletin tells technicians to purge air from the injector and its lines any time it is removed or replaced, and to run a forced regeneration afterward.
- Regen also depends on the engine reaching operating temperature, so a cooling system problem can show up as an exhaust code. A thermostat that sticks open is an inexpensive part to rule out before you replace exhaust components. Confirm actual coolant temperature with a scan tool, not just the dash gauge.
- For reference, P0420 is catalyst system efficiency, P2463 is DPF soot accumulation, and P24A0 is closed loop DPF regeneration control at limit.
Problem 3: The small stock turbo and power limits
Drivingline describes the LML turbo as the smallest and most restrictive variable-geometry unit offered on a Duramax. The tight turbine side lets GM use the turbo as an exhaust brake, which helps when towing. It also limits how far you can push the truck on tuning alone.
If you plan to add power:
- Fuel system first. Lift pump or CP3 conversion before a power tune.
- Transmission second. Drivingline notes the factory Allison needs attention beyond roughly 450 rear-wheel horsepower.
- Turbo and exhaust after that, once the fuel and drivetrain can handle it.
Chasing a power number before fixing the fuel system is the most common way an LML owner ends up with a very large repair bill.
Before you buy a used LML: checklist
- Confirm the engine code (LML vs LGH) and the model year. The 2015.5 split matters for exhaust parts (see below).
- Ask for fuel filter service history and any lift pump or CP3 conversion receipts.
- Scan for stored codes, especially P0087, P0088, P0191, P128E, P0420, P2463, and P24A0.
- Ask whether the truck has been tuned or had emissions hardware removed. A prior owner's changes affect what you can legally do with it.
- Check the warranty status with a GM dealer using the VIN.
Emissions hardware and compliance
Some LML owners ask about removing the DPF, EGR, or DEF system after repeated emissions faults. Some emissions-related products are intended only for off-road, race, or non-public-road use where permitted. An off-road label does not change the legal status of an EPA-certified vehicle. Check federal, state, and local laws before purchase, and the customer is responsible for legal compliance. Professional installation is recommended where appropriate.
If you are working through a repeated fault, start with diagnosis. Many LML codes trace back to the fuel system or the ninth injector, not the filter itself. Our LML Duramax delete kit buyer's guide explains fitment, what is included, and what tuning may be required.
Important fitment note: LML exhaust parts are split at the 2015.5 model year. Trucks built 2011 to 2015 connect the turbo to the downpipe with a V-band clamp. Trucks built 2015.5 to 2016 use a three-bolt flange. The two are not interchangeable without fabrication, and our kits are listed by the same split. Check your build date, look at the turbo outlet if you are unsure, and confirm cab and chassis trucks before ordering.
Important: Any work that removes or blocks the ninth injector requires its high-pressure fuel supply line to be capped or removed correctly, and the system checked for leaks after startup.
Frequently asked questions
What is the most common LML Duramax problem?
The Bosch CP4.2 high-pressure fuel pump. GM Service Bulletin 16-NA-102 documents hard start, no start, and codes P0087, P0088, P0191, and P128E on 2010 to 2016 LML and LGH trucks.
Does the LML have oil dilution from regeneration?
No. The LML uses a ninth injector in the downpipe, so regen fuel goes into the exhaust, not the cylinders.
Should I add a lift pump or convert to a CP3?
Both are common ways to protect or replace the CP4.2. A lift pump feeds the stock pump clean, pressurized fuel. A CP3 conversion removes the CP4.2. Ask your shop which fits your budget and your power plans.
Is the LML a bad truck to buy?
No. The engine is strong, and the main risk is known and can be managed. Check the fuel system history, scan for codes, and budget for a lift pump or conversion if the truck does not already have one.
Do I need a tuner for LML emissions parts?
Some emissions-related exhaust modifications may require tuning to address check-engine lights, diagnostic codes, or engine calibration. Confirm the full setup before the first bolt comes off.
Next step
Not sure what your LML needs? Send your year, engine code, cab style, bed length, and current setup, and EngineGo support will help you confirm fitment before you order.
