The Lemon Index
The 6.6 L V8 turbo diesel in the Chevrolet Silverado 2500HD/3500HD and GMC Sierra 2500HD/3500HD, model years 2004–2006. Owners filed 374 complaints about trucks with it (matched by VIN); 7% name the engine, #11 of the 12 diesel truck engine generations we track (1 = the most).
Known issues, checked against owner complaints
Problems owners of trucks with this engine name much more often than owners of other diesel truck generations (at least 1.5× the median, and 10+ complaints):
- Overheating when towing. 16 of 374 owner complaints (4.3%) mention coolant loss or overheating, against a median of 0.8% for the other diesel truck generations we track.
Instrument cluster failure, tachometer, and speedometer are inoperable, but all other gauges, such as fuel level, battery meter, oil pressure, coolant temperature, and transmission temperature gas are still functioning. own another 2005 1500 with 32,632 which has just started doing the same.
2005 owner · NHTSA complaint 10481278 - Injectors. 22 of 374 owner complaints (5.9%) mention an injector, against a median of 1.2% for the other diesel truck generations we track.
Failure of injector connector circuit on cylinder #7 failure code po207 - tsb #05-06-04-047c.
2004 owner · NHTSA complaint 10351947
What YouTube owners say Tone 0-100, higher = more praise than trouble.
Average owner tone 39/100 across 3 model years, from 148 videos about this engine and its cars.
Most-watched videos
- Duramax LLY - FICM / Injector Trouble / Misfire - Part 1 568,298 views
- CLEANEST 1 YEAR TRANSFORMATION OF MY DURAMAX TRUCK BUILD! | LLY | 543,603 views
- LLY DURAMAX TEARDOWN. BAD Silverado 6.6L Turbodiesel V8. Unrebuildable You Say? Yeah, Pretty Much. 500,425 views
Counted from video titles and captions: praise words against trouble words, weighted by views. Videos only count when the title names this engine or a car that had only this engine in those years.
Owner complaints by model year
Counts of complaints filed so far. Older trucks have had longer to collect them, and popular years draw more because more were sold: no one reports sales by engine, so there's no per-truck rate. Compare the shares below instead.
What owners complain about
Share of this engine's complaints that name each area (one complaint can name several).
What owners write
| Complaints that | Count | Share | Median, other diesel truck engines |
|---|---|---|---|
| Say the engine stalled, shut off or lost power | 29 | 7.8% | 7.8% |
| Mention an injector | 22 | 5.9% | 1.2% |
| Mention coolant loss or overheating | 16 | 4.3% | 0.8% |
Counted from the owner's own words in each complaint (all of the truck, not only the engine). A share, not a failure rate.
When problems show up
Half of the owners who gave a mileage reported the problem between 39,400 and 99,433 miles; the middle report was at 69,587 miles (345 complaints with a mileage).
Recalls that name this engine 0
No NHTSA recall on these trucks names this engine.
NHTSA recalls on these trucks and years whose text names this engine (its size, or diesel / Cummins / Duramax / Power Stroke). The rest (brakes, air bags, wiring ...) are on each truck year's page.
Trucks with this engine
In owners' words
The fan for the heater doesnt work on all speeds properly, stopped working entirely
2004 Chevrolet Silverado 2500HD/3500HD and GMC Sierra 2500HD/3500HD owner · NHTSA complaint 11544341
The brake line above the fuel tank blew out. it was not gradual either. the previous stop gave absolutely no warning. the next stop brake instantly went to the floor.
2005 Chevrolet Silverado 2500HD/3500HD and GMC Sierra 2500HD/3500HD owner · NHTSA complaint 11115767
Pulling out of a parking lot, lost brakes. steel brake lines by fuel tank, let go causing truck to loose brakes
2004 Chevrolet Silverado 2500HD/3500HD and GMC Sierra 2500HD/3500HD owner · NHTSA complaint 11082095
Against the other diesel truck engine generations
| Engine | Years | Complaints | Name the engine | Name the fuel system | Stalled or lost power | Recalls naming it |
|---|---|---|---|---|---|---|
| 6.0 Power Stroke | 2003–07 | 1,708 | 34% | 16% | 31% | 2 |
| 6.4 Power Stroke | 2008–10 | 572 | 28% | 7% | 11% | 2 |
| Duramax L5P | 2017–26 | 1,229 | 16% | 10% | 14% | 2 |
| 7.3 Power Stroke | 1999–03 | 2,049 | 15% | 5% | 12% | 1 |
| Duramax LML | 2011–16 | 779 | 13% | 20% | 17% | 0 |
| 6.7 Cummins | 2007–26 | 7,667 | 11% | 8% | 6.4% | 9 |
| 6.7 Power Stroke | 2011–26 | 5,640 | 10% | 11% | 7.8% | 6 |
| Duramax LBZ | 2006–07 | 186 | 8% | 5% | 7.0% | 0 |
| Duramax LMM | 2007–10 | 385 | 8% | 6% | 5.2% | 0 |
| Duramax LB7 | 2001–04 | 592 | 7% | 17% | 5.4% | 1 |
| Duramax LLY | 2004–06 | 374 | 7% | 11% | 7.8% | 0 |
| 5.9 Cummins | 1999–07 | 3,245 | 4% | 7% | 5.0% | 0 |
Shares of each engine's own complaints, matched by VIN. They show what owners complain about, not how often trucks break. Every diesel truck engine compared
Quick answers
Is the Duramax LLY reliable?
No one reports sales by engine, so there's no per-truck score. Of the 374 complaints owners filed about trucks with it, 7% name the engine, #11 of 12 diesel truck engine generations we track (1 = the most). 16 (4.3%) mention coolant loss or overheating, against 0.8% for its peers. 22 (5.9%) mention an injector, against 1.2% for its peers.
What trucks have the Duramax LLY?
In our data: the Chevrolet Silverado 2500HD/3500HD and GMC Sierra 2500HD/3500HD, model years 2004–2006, matched by the VIN in each owner complaint.
What are the most common Duramax LLY problems?
In owner complaints to the government: brakes, electrical, fuel system.
How many recalls name the Duramax LLY?
0 NHTSA recalls on these trucks name it.
Source: NHTSA owner complaints (FLAT_CMPL + COMPLAINTS_RECEIVED_2025-2026), received through 20261109; engines: NHTSA vPIC VIN decoder; NHTSA recalls API. Engine names and years: GM (RPO codes).