Data report

Duramax LML Problems: CP4 Pump Failure, DEF and What to Check (2011-2016)

Published September 26, 2026 · Updated September 29, 2026 · 13 min read

The Duramax LML in 2011-2016 Silverado and Sierra HD trucks has one problem that dwarfs the rest: its Bosch CP4.2 high-pressure fuel pump. When it fails, it sends metal through the whole fuel system, the engine dies, and the repair is rarely just a pump. A 2016 Sierra 3500 owner told NHTSA the bill was $16,145.

This guide shows how often that failure shows up by model year, what GM told its technicians, what to check on a truck you own or want to buy, and what the DEF system and the rest of the LML cost to fix. It is built from 212 NHTSA owner complaints that involve the diesel, GM's bulletins and special coverages, and current part prices, and it says where that evidence is thin.

The one idea: why a $2,300 pump becomes a $15,000 repair

The CP4.2 is a high-pressure pump driven by the engine. It feeds the fuel rails and the eight piezo injectors at very high pressure. When it wears internally, it sheds metal into the fuel it is pumping, and that fuel goes everywhere. aggregated

1. The pump wears inside aggregatedShops and owners point to fuel with poor lubricity, water or air in the fuel. Bosch and GM have not published a cause.
2. Metal goes into the high-pressure side verifiedGM told technicians to inspect the fuel pressure regulator for magnetic metal debris before replacing a pump (PIP4949).
3. Rails and injectors are contaminated verifiedGM's contamination guide says that if contamination is found, the complete fuel system has to be cleaned and flushed (PIP5151).
4. The engine dies verifiedRail pressure collapses. 37 of the 75 CP4 complaints describe a stall, a sudden loss of power or the engine shutting off, often at highway speed.

That is why owners report bills five or more times the price of the pump: pump, injectors, rails, lines and a cleaned tank.

What Duramax LML owners report to NHTSA

The CP4 pump accounts for 75 of the 127 complaints that name a specific engine problem. 47 of them mention metal, contamination or the whole fuel system. The DEF system is a distant second at 29. Everything else, from EGR to glow plugs, is in single digits.

Two more patterns: 65 of the 75 CP4 complaints were filed in 2019 or later, as these trucks aged past warranty and the pump's reputation spread. And some owners filed before anything failed, asking GM to replace the pump before it does.

A note on how we counted: GM sold the same trucks with a 6.0 gas V8. We kept complaints that mention the diesel or a diesel-only part and dropped those that name the gas engine. Some diesel complaints that name neither are left out.

LML Duramax years to avoid: what the data shows by year

Short answer: every LML year has the CP4 pump, so no year avoids the risk. CP4 complaints are heaviest on 2015-2016 trucks (25 and 21) and lightest on 2013 (3). DEF complaints cluster on 2011 trucks (12 of 29), the first year of the system. The 2015 body change did not change the engine.

Complaints: NHTSA owner complaints filed on 2011-2016 Chevrolet Silverado and GMC Sierra 2500/3500 HD trucks, pulled 2026-09-29. Raw counts, not adjusted for how many trucks were sold or how old they are.

How to read this table. It counts complaints, not failures, and we cannot adjust for how many trucks of each year were sold or for their mileage. One more signal points the same way as the counts: in 2019 GM engineering asked dealers for information on 2015-2016 trucks that had had their fuel pump replaced under warranty (PIE0523). verified

Between 2016 and 2018 GM also put the LML's pump part numbers on a parts restriction, so a dealer had to contact GM's Product Quality Center before replacing one (PIP5133), and lifted it in June 2018. verified That is engineering follow-up, not a recall. There is no safety recall for the CP4 on these trucks in NHTSA's records.

CP4 pump failure on the LML: the symptoms

What owners describe. A sudden loss of power, rough running, sometimes a faint noise from the engine, then a stall that will not restart. Some saw a warning first:

"I received a warning from my dashboard saying fuel filter needs replacing and subsequently lost engine power which resulted in no power steering and no brakes."

Owner of a 2012 Silverado 2500, 2016. The repair was $8,550. NHTSA complaint 10873931

The codes. GM's bulletins on hard starts, no-starts and contamination name P0087 (fuel rail pressure too low), P0088 (too high), P0191 (rail pressure sensor performance) and P128E. verified These codes have other causes too, from a clogged filter to a weak lift pump, which is why GM warned in 2012 that normal diagnostics "may be inconclusive or lead to a fuel injection pump replacement" (PIP4949B).

If it dies on you. Stop cranking. A pump that has started to come apart pushes more debris through the system on every crank. Have it towed and ask the shop to check the fuel filter and the pressure regulator for metal before anything else. aggregated

Duramax LML "starts then dies": what it can be

Short answer: a CP4 failure is the expensive possibility, but not the only one. A clogged fuel filter, air or water in the fuel, or a weak in-tank lift pump can cause the same low-rail-pressure stall. On 2015-2016 trucks with dual fuel tanks, GM issued an emissions recall (17067) because fuel could transfer too slowly from the rear tank: the truck sets P2636, the gauge reads empty, and it can run the front tank dry and stall. verified

Check the cheap causes first, but check the filter for metal before replacing parts: that tells you which repair you are facing.

What a CP4 failure costs on the LML

"CP4 high pressure fuel pump failed/disintegrated causing metal shavings to be pumped through out the fuel lines, injectors and connected systems."

Owner of a 2014 Sierra 3500 at 94,112 miles, 2021. The repair was over $11,000. NHTSA complaint 11440947

Owner-reported bills for a CP4 failure on LML trucks:

• $7,000 to $10,000 in complaints filed 2016-2018.
• $8,000 to $17,000 in complaints filed 2020-2025, most of them $10,000 or more, including $13,520 (2016 Sierra 2500, 2025), $16,145 (2016 Sierra 3500, 2023) and $17,000 (2016 Sierra 3500, 2024).

At list prices, GM's high-pressure pump service kit is about $2,315, injectors about $237 (remanufactured) to $366 each, eight per engine, and the fuel rails about $344 and $378. Labor and the clean-out make up the rest. One 2016 owner paid $6,500 to a shop, then needed the repair redone at a dealer. Ask what the job includes: pump only, or pump, injectors, rails, lines and tank.

Is there a recall or extended warranty? Not for the CP4 on the LML in NHTSA's records. GM's 5-year/100,000-mile powertrain warranty on these trucks has run out on every one of them. aggregated

How to check a used LML for CP4 trouble

• Ask for fuel filter records. On-schedule filter changes and draining the water separator are the cheapest protection the system has. aggregated
• Ask whether the pump has been replaced, and if so, what else was replaced and by whom. A pump-only repair on a contaminated system can fail again.
• Scan for codes, stored and pending: P0087, P0088, P0191, P128E and P2636 on dual-tank 2015-2016 trucks.
• Watch for a "change fuel filter" message that appears on a truck whose filter is new.
• If you are serious, pay a diesel shop to pull the fuel filter and look for metal. It is cheap compared with what it can save you.

What you cannot do is inspect the pump's internal wear from outside. A truck with a clean filter today can still fail later. Price that risk into your offer.

DEF system problems on the LML (and GM's special coverage)

"My DEF heater just went out and my dealership said it's only under warranty until 120K miles."

Owner of a 2011 Silverado 2500, 2020. NHTSA complaint 11306829

What you will see. A "Service DEF System" message, a countdown to a speed-limited mode, and codes for the DEF tank heater or temperature sensor (one owner reported P20B9, reductant heater circuit open). verified

GM's special coverages. GM extended coverage on the DEF tank reservoir for these trucks:

• 14652, 2011-2012 Silverado and Sierra HD: a temperature sensor offset in the DEF tank reservoir, higher tank-heater resistance or reductant system performance can light the check-engine light; dealers replace the DEF tank reservoir and/or the supply module pump. verified
• 29400, 2013-2015 Silverado and Sierra HD: the same reservoir sensor and heater problem, covered for 10 years or 120,000 miles from the date the truck was first put in service, whichever comes first. verified By now that has run out on most of these trucks.

What it costs. Owners reported $1,200 to over $2,000 for DEF repairs in 2018-2022 complaints. At list prices, a DEF tank reservoir kit is about $215 (2012-2016) to $350 (2011), the DEF pump module kit about $466 and the DEF tank assembly about $285.

Exhaust: the downpipe leak and the regeneration injector

Two more GM special coverages involve the exhaust side of the LML: verified

• 15457, 2011-2015 Silverado and Sierra HD: the seal between the turbo downpipe and the exhaust pipe can leak. During DPF regeneration, small amounts of diesel fuel can leak out, and fuel vapor and exhaust can enter the cab. Dealers inspect the joint and fit a service kit.
• 14653, 2011-2014 Silverado and Sierra HD: the indirect fuel injector, which sprays fuel into the exhaust to heat the DPF for regeneration, can stick closed or flow too little and light the check-engine light. Dealers replace it with an improved part. That injector lists for about $208.

If you smell diesel in the cab during or after a regeneration, have the downpipe joint checked.

The rest: EGR, DPF, injectors, glow plugs

These show up far less often in the complaints:

• EGR valve and cooler (6 complaints). Carbon builds up in the valve and the cooler can leak coolant. List prices are about $339 to $350 for the valve and $376 to $513 for the cooler, by year. aggregated
• DPF (6). Short trips and idling load the filter faster than it can regenerate. A replacement filter lists for about $2,290 to $2,580 by year. aggregated
• Injectors (3). Usually replaced as part of a CP4 repair rather than on their own.
• Glow plugs and control module (3). Hard starts in the cold. Glow plugs list for about $25 each and the control module $258 to $279.

What you found, and what to do about it

Part prices are GM list prices at the time of publication; the aftermarket is often cheaper. Owner-reported costs come from NHTSA complaints and show what those owners paid or were quoted in those years, not a quote. We do not have reliable labor estimates for these jobs.

What we cannot tell you

• The CP4 failure rate. 75 complaints out of hundreds of thousands of trucks says nothing about your truck's odds. We have no sales or warranty data.
• Why 2015-2016 trucks draw more complaints. The counts and GM's 2019 data request point the same way, but we do not know whether the cause is the pump, the trucks' use or simply when owners started reporting.
• Whether a truck that has not failed will fail. A clean filter today does not prove the pump is healthy inside.
• Whether additives or filters prevent it. Owners and shops recommend them; we have no data that measures their effect.
• Labor costs. We quote part list prices and what owners reported, not shop rates.

FAQ

Go deeper

Each of these pages shows the sources behind every claim, the confidence tier and the part numbers:

• Duramax LML platform page · the engine that replaced it: Duramax L5P · Duramax LML vs L5P, side by side
• Failure modes: CP4 pump and fuel contamination · DEF system · DPF and regeneration · EGR · Injectors · Glow plugs
• Repairs: CP4 contamination repair · DEF system repair · Fuel system health
• Parts: High-pressure pump service kit · Injector · DEF reservoir kit · Regeneration fuel injector
• How we verify and tag facts

Sources. NHTSA owner complaints database (US government public domain), keyword-matched and limited to complaints that involve the diesel; owner quotes are taken from complaint narratives with capitalization normalized, and complaint numbers are given so you can read them in full. GM bulletins PIP4949, PIP5133, PIP5151 and PIE0523, special coverages 14652, 29400, 14653 and 15457, and emissions recall 17067, from NHTSA's manufacturer communications index. NHTSA recall records checked for these trucks. Part prices are GM list prices at the time of publication. Torque Index aggregates and cross-checks public information, takes no paid placement and does not provide repair or inspection services.