For fifteen years, the OM642 was Mercedes' 3.0-litre diesel V6 — in the E-Class, the ML, the Sprinter, half the range. Then in 2017 Mercedes did something that looked like going backwards: it replaced the V6 with a straight-six, the OM656. Same displacement, fewer banks, older layout.
It wasn't nostalgia. It was engineering. And the two engines' fault profiles — identical 82/100 scores, completely different failure modes — explain exactly why the switch made sense.
Quick answer: Both score 82/100 (BUY). The OM642 is the value pick if you accept its one signature weakness — the "valley" oil cooler seal (€800–2,000). The OM656 trades that mechanical risk for cleaner running and a simpler, more serviceable layout. Newer money buys the OM656; smart money on a budget buys a well-sorted OM642.
The architecture question
A V6 packs two cylinder banks into a short, wide engine. It's compact — but it buries components in the valley between the banks, where heat collects and access is poor. A straight-six is longer, but everything lines up on one side, runs smoother by nature, and is far easier to work on. Mercedes' fault data shows that trade-off perfectly.
OM642 — 82/100 (BUY)
The V6 (2005–2020, ~265 hp) is a strong, willing engine with one expensive Achilles' heel that every owner learns to fear:
| Problem | Severity | Repair cost |
|---|---|---|
| Valley oil cooler seal failure (the #1 issue) | High | €800–2,000 |
| Intake swirl flap actuator failure | Moderate | €400–900 |
| BlueTEC NOx / DEF system (where fitted) | Moderate | €300–1,500 |
That top item is pure V6 geometry. The oil cooler sits in the valley between the cylinder banks; when its seals harden, oil weeps into the V, and the repair means going in deep. It's the single most-quoted OM642 job in any indie workshop.
Found in: Mercedes C-Class, E-Class (E 350d), S-Class, CLK, GLE (GLE 350d), ML 350 CDI, G-Class.
OM656 — 82/100 (BUY)
The straight-six (2017→, ~330 hp) is the cleaner-sheet design. No valley to trap an oil cooler, smoother running, more power — and its issues are the ordinary emissions-era items rather than a deep mechanical repair:
| Problem | Severity | Repair cost |
|---|---|---|
| DPF + EGR (diesel city-driving concern) | Moderate | €400–1,200 |
| AdBlue / SCR system faults | Minor | €100–800 |
Same score as the OM642 — but notice there's no high-severity line. The OM656's risks are about emissions plumbing, not buried hardware.
Found in: Mercedes E 400d, GLE 350d, S 350d.
The trade, in one table
| OM642 (V6) | OM656 (I6) | |
|---|---|---|
| Layout | V6 | Straight-six |
| Years | 2005–2020 | 2017→ |
| Power | ~265 hp | ~330 hp |
| Score | 82/100 | 82/100 |
| Defining fault | Oil cooler in the valley (€2,000) | DPF/EGR (€1,200) |
| Serviceability | Tight | Easy |
The verdict
It's a genuine tie on score — so this comes down to what kind of risk you'd rather own.
- Buy the OM642 if you want a proven, cheaper V6 and you're willing to either pre-empt the oil-cooler seal or buy a car that's already had it done. Get that one job sorted and it's a long-haul engine.
- Buy the OM656 if you want the newer, smoother, more powerful straight-six with no single scary mechanical job lurking — just the standard modern-diesel emissions maintenance.
Mercedes went back to the straight-six because it's the better engine to build, run and fix. The data agrees — even if the scores, fittingly, came out even.
For the four-cylinder version of this story, see OM651 vs OM654.
Scores are calculated from documented issues, repair costs, and failure patterns. Reviewed by the EngineScope editorial team. Methodology →