The OM651 2.1 diesel generated more UK replacement enquiries in 2025 than any other Mercedes engine — 1,250 in total. The full ranking, real cost data, and what it means if you own one.
Across 4,500+ real UK Mercedes engine enquiries in 2025, the OM651 2.1 diesel leads at 1,250 requests, followed by the OM642 V6 diesel at 880 — together accounting for nearly half of all Mercedes engine enquiries we handled last year.
Full methodology, the complete ranking, and what changed in this baseline year below.
What this data is, what it isn't, and how we built it.
Source: Internal CRM quote-request data from EnginesMarket.co.uk and EngineFinders.co.uk, combined. Full calendar year 2025. 24,650 unique quote requests across all brands we cover; over 4,500 of these relate specifically to Mercedes engines.
What counts as an "enquiry": a unique customer quote request specifying a particular engine code or vehicle. Repeat requests from the same customer for the same engine are not double-counted.
What this data cannot tell you: our dataset reflects people who contacted us about a replacement engine — it is not a random sample of every Mercedes on UK roads. A higher enquiry count can mean an engine is more failure-prone, or simply that more of that engine exist in the parc, or that it's an older engine reaching typical replacement age. We do not claim failure rates against the total vehicle population; only enquiry share within our own dataset.
Refresh cycle: annually, each February, covering the prior full calendar year. Figures are locked at time of publication and are not silently revised — any correction is logged on our public corrections page rather than edited away.
The engines that generate the most enquiries, and what it means.
All figures [EM-VERIFIED], full year 2025.
The OM651 is the most-enquired Mercedes engine by a significant margin — 1,250 requests, well ahead of the OM642 at 880. This reflects the OM651's sheer popularity across C‑Class, E‑Class, GLA, and Sprinter vehicles, as well as its known weak points: timing chain tensioner wear and injector seal leaks. The OM654, a newer 2.0 diesel, already sits at 520 enquiries, indicating that as the vehicle parc ages, we expect this number to grow.
Every Mercedes engine in our 2025 enquiry data, ranked, with real cost and vehicle-value context.
| Rank | Engine | 2025 Enquiries | Recon Supply & Fit | Typical Host Value | R-to-V Flag |
|---|---|---|---|---|---|
| 1 | OM651 | 1,250 [EM-VERIFIED] | £3,500–£5,500 | £5,000–£18,000 | ✅ Safe |
| 2 | OM642 | 880 [EM-VERIFIED] | £4,000–£6,500 | £5,000–£22,000 | ✅ Safe |
| 3 | OM654 | 520 [EM-VERIFIED] | £3,700–£5,800 | £8,000–£30,000 | ✅ Safe |
| 4 | M276 | 390 [EM-VERIFIED] | £4,800–£7,800 | £8,000–£35,000 | ✅ Safe |
| 5 | M272 | 390 [EM-VERIFIED] | £4,800–£7,800 | £8,000–£35,000 | ⚠️ Watch |
| 6 | M177 | 370 [EM-VERIFIED] | £9,000–£15,500 | £30,000–£120,000 | ✅ Safe |
| 7 | M274 | — [ESTIMATED] | £3,700–£5,800 | £8,000–£30,000 | ✅ Safe |
| 8 | OM656 | — [ESTIMATED] | £6,000–£9,500 | £15,000–£55,000 | ✅ Safe |
| 9 | M273 | — [ESTIMATED] | £5,000–£8,000 | £8,000–£35,000 | ⚠️ Watch |
All enquiry figures [EM-VERIFIED]. Cost and value bands [EM-QUOTE / EM-INTERVIEW / THIRD-PARTY] per our data legend — full source detail on the Data Verification page.
This is our first edition — establishing the baseline for future comparisons.
As we are publishing this report for the first time in 2026, there is no prior year to compare against. However, we have built a robust methodology that will allow us to track changes year on year — for example, whether the OM654 starts to overtake the OM651 in enquiry volume as more C‑Class and GLC models age into the replacement window.
In future editions, we will highlight shifts such as:
For now, this report serves as the definitive benchmark for Mercedes engine failure in the UK, based on our 2025 enquiry data.
How to use this data when buying, maintaining, or repairing a Mercedes.
The two most-enquired engines, OM651 and OM642, are not necessarily the two "worst" Mercedes engines — they are simply the most common ones reaching typical replacement mileage across a large vehicle parc. What's more informative is the R-to-V (repair‑to‑value) column: almost every engine on this list clears our 70% safety threshold, meaning replacement is financially sound on the typical host vehicle.
The engines flagged ⚠️ Watch — M272 and M273 — are older V6 and V8 units where the cost of replacement can be a significant portion of the vehicle's value. On sub‑£8k W204 or W211 examples, it's worth checking the decision carefully using our Repair vs Replace Calculator.
For the full breakdown behind any single figure on this page, see Failure Rate by Engine, Average Mileage at Failure, and Most Enquired Models.
Feel free to reference this data in press coverage, forum discussion, or your own research. Suggested attribution:
For press enquiries or the full underlying dataset, contact us via our About page. We're happy to provide additional context or breakdowns for journalists working on a related story.
The OM651 2.1 diesel leads with 1,250 enquiries in our 2025 data, followed by the OM642 V6 diesel at 880. This reflects both engines' popularity across the range, not a claim that either is unreliable by design [EM-VERIFIED].
Not on its own. Enquiry volume reflects how many vehicles are on the road with that engine as much as it reflects failure frequency. We pair enquiry data with our separate Known Failure Patterns research to distinguish popularity from reliability [PATTERN DATA].
No, and we want to be upfront about that. Our dataset reflects people who contacted us about a replacement engine, which naturally over-represents engines that fail and under-represents ones that don't. Full methodology and limitations are set out in this report.
Annually, each February, covering the prior full calendar year. This is our first published edition and establishes the baseline every future report will be measured against.
Our 70% threshold means a replacement is financially sound if the fitted cost is less than 70% of the vehicle's current market value. For example, a £4,500 engine in a £7,000 C‑Class is borderline; a £6,000 engine in an £8,000 E‑Class is a decision point [PATTERN DATA].
The OM651 is a robust diesel when maintained well, but it has known weak points: timing chain tensioner wear and injector seal leaks (Black Death). Many examples exceed 200,000 miles, but the failures that do occur tend to happen between 80,000 and 150,000 miles [PATTERN DATA].
4,500+ Real Enquiries The full Mercedes-specific dataset behind every figure on this page.
Limitations Disclosed We state plainly what this data can't tell you, not just what it can.
Annual Publication Refreshed every February — this is our founding baseline year.
100% Independent No sponsors, no manufacturer influence. Part of the Engine Finders network.
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