Where people disagree
Power Stroke vs Cummins vs Duramax: What Owners Report, System by System (No Rankings)
Published · Updated · 7 min read
Every "which diesel is most reliable" list picks a winner, and almost none of them can show the data that would justify it. We have 4,543 NHTSA owner complaints on ten diesel engines from Ford, Ram and GM, and they cannot justify a winner either. What they can show is different and more useful: which system goes wrong on each engine, so you know what to check on the truck in front of you.
This report sorts each engine's complaints into the system they name: fuel, DEF, DPF, EGR, turbo, cooling, cold start, sensors or engine internals. Each engine links to our full article on it.
The short answer
- No ranking: we have no sales or mileage figures, the engines span 30 years, and owners report some failures far more than others. Complaint counts cannot say which brand is more reliable, and we do not pretend they can.
- The fuel system leads on most engines: it is the largest named system on seven of ten, from the 6.0 Power Stroke to the 2019+ Cummins. On the CP4-pump engines (6.7 Power Stroke, LML, 2019+ Cummins) it is 28% to 44% of complaints.
- The exceptions say the most: the 7.3 is about a sensor (the cam sensor), the 2007.5-2018 6.7 Cummins about its turbo actuator, and the L5P Duramax about the DEF system.
- Emissions hardware changed the picture: DEF complaints appear from 2011 and reach 29% on the L5P; EGR is the 6.0's story (11%).
- What to do with it: pick the truck, then check the system its engine is known for. Each engine's article below has the inspection route.
Why we will not rank Power Stroke, Cummins and Duramax
Three reasons, and each one alone would be enough:
- No denominator. A complaint count means nothing without the number of trucks and miles behind it. The brands sold different numbers of diesel trucks each year, and a brand with more trucks on the road collects more complaints about everything. We do not have sales or mileage by engine.
- Owners report what scares them. A stall on the highway or a speed-limit countdown gets reported to a safety regulator; a slow oil leak or worn turbo seal rarely does. So engines whose failures are sudden look worse than engines whose failures are slow.
- The sets are not alike. The engines span 1994 to 2026. NHTSA holds few complaints from the 1990s, and the newest trucks have barely started collecting them. Each set was built for its own article, with filters that fit that engine.
What we can compare is the structure inside each engine: of the complaints that one engine drew, what share names each system. That is what the table shows.
What owners report, engine by engine
| Engine | Complaints | Fuel system | DEF / SCR | DPF / regeneration | EGR | Turbo / boost | Cooling / head gasket / oil cooler | Glow plugs / grid heater | Sensors and electronics | Engine internals | No part named |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 7.3 Power Stroke (1994.5-2003) | 607 | 21% | 0% | 0% | 0% | 2% | 2% | 0% | 25% | 3% | 47% |
| 6.0 Power Stroke (2003-2007) | 980 | 35% | 0% | 0% | 11% | 11% | 4% | 1% | 3% | 1% | 34% |
| 6.4 Power Stroke (2008-2010) | 159 | 28% | 0% | 10% | 4% | 8% | 9% | 0% | 1% | 3% | 36% |
| 6.7 Power Stroke (2011-2026) | 666 | 36% | 13% | 7% | 0% | 3% | 1% | 0% | 1% | 2% | 38% |
| 5.9 Cummins (1994-2007) | 477 | 23% | 0% | 1% | 0% | 1% | 3% | 0% | 3% | 7% | 62% |
| 6.7 Cummins (2007.5-2018) | 656 | 3% | 8% | 8% | 2% | 16% | 4% | 3% | 1% | 3% | 53% |
| 6.7 Cummins (2019-2025) | 318 | 28% | 9% | 7% | 1% | 4% | 1% | 7% | 0% | 4% | 40% |
| Duramax LB7-LMM (2001-2010) | 283 | 25% | 0% | 2% | 0% | 2% | 7% | 1% | 1% | 2% | 59% |
| Duramax LML (2011-2016) | 212 | 44% | 15% | 2% | 2% | 0% | 0% | 1% | 1% | 0% | 33% |
| Duramax L5P (2017-2026) | 185 | 24% | 29% | 4% | 1% | 4% | 4% | 3% | 1% | 2% | 29% |
Share of each engine's own NHTSA complaints, one system per complaint (first match: fuel, DEF, DPF, EGR, turbo, cooling, cold start, sensors, internals). Bold: the largest named system for that engine. Read across a row, never down a column: engines differ in sales, age and how their complaints were collected, so the counts do not rank reliability. Sets from our articles on each engine, pulled 2026-09-08 to 2026-10-16.
How to read it. Read across a row. "28% fuel system" on the 6.4 means 28% of the 6.4's own complaints name the fuel system first; it does not mean the 6.4's fuel system fails more than the Duramax's. "No part named" includes complaints about the rest of the truck and stalls where the owner did not know the cause.
Ford Power Stroke: four engines, four stories
- 7.3 (1994.5-2003): the cam sensor. Sensors and electronics lead at 25%, almost all the camshaft position sensor that Ford recalled in 2007. The fuel system is next, including fuel tanks whose lining came apart on heavier trucks and vans. Read: buying a used 7.3 · 7.3 won't start
- 6.0 (2003-2007): fuel, EGR and turbo. The fuel system (35%, including the injector control module), the EGR system (11%) and the turbo (11%). Read: buying a used 6.0
- 6.4 (2008-2010): fuel first, then the DPF and cooling. The high-pressure pump and contaminated fuel (28%), the DPF and its regeneration (10%) and the radiator (9%). Read: buying a used 6.4
- 6.7 (2011-2026): the CP4 pump and DEF. The fuel system leads at 36%, then DEF at 13%. Read: 6.7 Power Stroke problems by year
Ram Cummins: the 6.7 is the outlier
- 5.9 (1994-2007): the fuel system across three designs. 23% fuel: the VP44 and lift pump on the 24-valve, injectors on the common rail. A high share of complaints name no part, which is typical of the oldest data. Read: buying a used 5.9 · VP44 failure
- 6.7 (2007.5-2018): the turbo, not the fuel. The only engine here whose fuel system barely registers (3%). The turbo and its electronic actuator lead at 16%, with DPF and DEF at 8% each. Read: 6.7 Cummins problems · buying a used 6.7 Cummins
- 6.7 (2019-2025): back to fuel. The updated engine switched to the CP4 pump on its first years, and the fuel system returns to the top at 28%; the grid heater relay shows up in cold-start complaints (7%). Read: 2019+ Ram 6.7 problems
GM Duramax: injectors, then CP4, then DEF
- LB7-LMM (2001-2010): injectors. 25% fuel system, mostly LB7 injectors, plus LLY overheating (7% cooling). Read: buying a used Duramax LB7-LMM
- LML (2011-2016): the CP4. The highest fuel system share of any engine here, 44%, most of it the CP4 high-pressure pump, plus DEF at 15%. Read: Duramax LML problems
- L5P (2017-2026): DEF. The only engine where DEF (29%) outranks the fuel system (24%). The L5P dropped the CP4. Read: Duramax L5P problems
Three patterns that cross brands
- The CP4 era is visible. The three engines that used the Bosch CP4 high-pressure pump in this period (6.7 Power Stroke, LML Duramax, 2019+ Cummins in its early years) all show the fuel system at 28% to 44%. Read: which trucks have the CP4 verified
- DEF arrived with problems. No DEF complaints exist before 2011, because there was no DEF. Since then it is 8% to 29% of complaints on every engine that has it. Read: DEF warning and 5 mph derate
- The engine internals rarely appear. Blocks, pistons, rods and valvetrain are 0% to 7% on every engine. What strands owners is the equipment around the engine.
What we cannot tell you
- Which brand or engine is most reliable. Without sales and mileage, nobody can say it from complaints, and we will not.
- How often any of these systems fails. These are shares of complaints, not failure rates.
- Exact system splits. Each complaint gets one system, the first that matches; a complaint about a pump that caused a stall counts as fuel. Our engine articles count mentions instead and give the details.
- The newest trucks. 2025-2026 trucks have barely started collecting complaints.
FAQ
Which is more reliable: Power Stroke, Cummins or Duramax?
Public complaint data cannot answer it, because it has no sales or mileage behind it and owners report sudden failures far more than slow ones. What the data shows is which system each engine tends to have trouble with: for example the CP4 fuel pump on the LML Duramax and 6.7 Power Stroke, the turbo actuator on the 2007.5-2018 6.7 Cummins, DEF on the L5P.
What is the most reliable diesel truck engine?
No honest answer exists from complaint data. Rankings you see online rest on shop opinions or whole-vehicle surveys. In NHTSA complaints the engine internals of every modern diesel rarely fail; the fuel system, emissions hardware and sensors around them do. Pick a truck, then check the system its engine is known for.
Duramax vs Power Stroke: which has more problems?
Counts cannot say, because the brands sold different numbers of trucks. Structurally, the LML Duramax and 6.7 Power Stroke look alike (both led by the CP4 fuel pump), while the L5P Duramax's complaints are led by DEF and the 6.0 Power Stroke's by fuel, EGR and turbo.
Which diesel trucks have the CP4 pump?
In this period: the 6.7 Power Stroke (2011 on), the LML Duramax (2011-2016) and the 2019-and-later 6.7 Cummins in its early years. The L5P Duramax uses a different pump. Our CP4 article lists the trucks by year.
Which diesel has the most DEF problems?
As a share of its own complaints, the L5P Duramax (29%), followed by the LML Duramax (15%) and the 6.7 Power Stroke (13%). That is a share within each engine, not a rate across engines.
Go deeper
- Ford: 7.3 · 6.0 · 6.4 · 6.7
- Ram: 5.9 · 6.7 (2007.5-2018) · 6.7 (2019+)
- GM: LB7-LMM · LML · L5P
- Platforms: 7.3 · 6.0 · 6.4 · 6.7 Power Stroke · 5.9 Cummins · 6.7 Cummins · Duramax L5P
- How we verify and tag facts
Sources. NHTSA owner complaints database (US government public domain). Each engine's set comes from our article on that engine and was keyword-matched (and, for the newest reports, read by hand); here each complaint is assigned to the first system it names. We do not publish reliability rankings: we have no sales or mileage data, and Torque Index takes no paid placement from any brand or seller.