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 — 65/100 against 82/100, completely different failure modes — explain exactly why the switch made sense.
Quick answer: The OM656 scores 82/100 (BUY), the OM642 65/100 (CAUTION). The OM642 stacks several expensive faults — main-bearing failure on early engines, the "valley" oil cooler seal (up to €2,000) and the swirl-flap system (€2,000–2,800). The OM656 trades that mechanical risk for cleaner running and a simpler, more serviceable layout — just check its coolant-pump recall was done. Buy the OM656 if you can; buy an OM642 only with paperwork.
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 — 65/100 (CAUTION)
The V6 (2005–2020, ~265 hp) is a strong, willing engine with more expensive weak points than its reputation suggests:
| Problem | Severity | Repair cost |
|---|---|---|
| Crankshaft main bearing failure (mainly up to MY 2009, 150,000–200,000 km) | Critical | Rebuild or exchange engine |
| Valley oil cooler seal failure | High | up to €2,000 |
| Swirl flap / intake port shut-off (EKAS) failure | High | €2,000–2,800 |
| EGR valve and cooler sooting | Moderate | — |
| BlueTEC NOx / emissions software recall | Moderate | €0 (recall) |
The oil cooler is pure V6 geometry. It sits in the valley between the cylinder banks; when its seals harden, oil weeps into the V (or into the coolant), and the repair means going in deep. The bigger risk on early engines is the main bearings: a rebuilder calls them the most common cause of total OM642 failure. Listen for knocking at idle and check the oil filter for metal.
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 apart from one recall its issues are the ordinary emissions-era items rather than a deep mechanical repair:
| Problem | Severity | Repair cost |
|---|---|---|
| Coolant pump leaks into the vacuum circuit — recall (action 2090008) | High | €0 (recall) |
| DPF + EGR (diesel city-driving concern) | Moderate | €400–1,200 |
| AdBlue / SCR system faults | Minor | €100–800 |
Seventeen points ahead of the OM642. The OM656's one high-severity line is a recall Mercedes fixes free, so confirm it was done; the rest is 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 | 65/100 | 82/100 |
| Defining fault | Main bearings (early), oil cooler in the valley (€2,000) | DPF/EGR (€1,200) |
| Serviceability | Tight | Easy |
The verdict
The OM656 wins clearly on score — the OM642 makes sense only as the cheaper buy, with the right history.
- Buy the OM642 if you want a cheaper V6 and can find a later car (2010 on) with the oil-cooler seals and swirl flaps already done and no knocking at idle. With those jobs sorted it can still be 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 coolant-pump recall to confirm and 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.
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 →