There are two ways to read the story of Mercedes' four-cylinder diesels. One is a cautionary tale. The other is a comeback. The OM651 is both — depending entirely on which year you buy — and the OM654 is the engine that let Mercedes put the whole episode behind it.
If you're shopping a used C-Class, E-Class or GLC diesel, this is the single most important number on the windscreen: the year. Get it wrong and you inherit a four-figure injector bill. Get it right and you own one of the best small diesels ever built.
Quick answer: The OM654 (2016→) is the clear buy at 85/100. The OM651 is a trap only in its early form — pre-2014 cars score 38/100 (AVOID) on injectors and timing chain. Post-2014 OM651s recover to a respectable 72/100. Never buy an OM651 without knowing its build year and injector history.
Act one: how the OM651 went wrong
The OM651 arrived in 2008 as Mercedes' new mainstream 2.1-litre diesel, and it brought two expensive habits with it.
OM651 (early) — 38/100 (AVOID)
The pre-2014 cars are the problem children. Two failures define them, and both are wallet-emptying:
| Problem | Severity | Repair cost |
|---|---|---|
| Piezoelectric injector failure | Critical | €1,200–3,500 |
| Timing chain stretch | Critical | €1,000–2,500 |
| DPF + EGR fouling (city use) | High | €500–1,200 |
Two critical faults on one engine is rare in our database — and it's why the early OM651 sits at the bottom of the modern Mercedes range. The injectors were the headline act: when they go, you're often replacing all four.
Found in: Mercedes A-Class, B-Class, C 200d, C 220d, E 200d, E 220d, CLA, GLA, GLC, GLE, GLK (2008–2014).
OM651 (late) — 72/100 (ACCEPTABLE)
Here's the nuance most buyers miss. Mercedes quietly revised the OM651 around 2014, and the later cars are a different proposition — the critical injector and chain issues recede, leaving only the diesel-universal DPF and EGR concerns:
| Problem | Severity | Repair cost |
|---|---|---|
| DPF blockage (city driving) | High | €500–1,500 |
| EGR valve fouling | Moderate | €300–700 |
A 34-point swing on the same engine name. This is why "is the OM651 reliable?" has no single answer.
Found in: Mercedes A 180d, B 180d, C 200d, C 220d, E 200d, E 220d, GLA 200d, GLC 220d (2014–2020).
Act two: the OM654 redemption
In 2016 Mercedes threw out the architecture and started again — an all-aluminium block, stepped-bowl combustion, and a clean-sheet design. The result is the quietest, cleanest diesel the company has ever sold to the public.
OM654 — 85/100 (BEST)
The contrast with the early OM651 is stark. No critical faults. Nothing left but the unavoidable realities of running any modern diesel:
| Problem | Severity | Repair cost |
|---|---|---|
| DPF (better than older MB, still a diesel) | Minor | €300–1,000 |
| EGR fouling (best-in-class, not immune) | Minor | €200–500 |
Found in: Mercedes A 180d, C 220d, C 300d, E 220d, GLC 220d, GLE, S-Class (2016–present).
The verdict
| OM651 early | OM651 late | OM654 | |
|---|---|---|---|
| Score | 38/100 | 72/100 | 85/100 |
| Verdict | AVOID | ACCEPTABLE | BEST |
| Years | 2008–2014 | 2014–2020 | 2016→ |
| Critical faults | 2 | 0 | 0 |
The OM654 wins, and it isn't close. But the more useful takeaway is about the OM651: the badge tells you nothing — the build date tells you everything. A 2012 C 220d and a 2018 C 220d can wear the same engine family name and live in completely different reliability universes.
What to do at the dealer
- Buying an OM654? Just confirm regular servicing and that the DPF regenerates (motorway miles, not pure city use). It's a safe car.
- Tempted by a cheap OM651? Check the year first. Pre-2014: walk away unless you've seen proof the injectors and timing chain were replaced. Post-2014: acceptable, budget for DPF/EGR.
- Cross-shop the OM654 against BMW's B47 in our OM654 vs B47 comparison.
Scores are calculated from documented issues, repair costs, and failure patterns. Reviewed by the EngineScope editorial team. Methodology →