The OM642 is a 3.0‑litre V6 diesel, while the OM651 is a 2.1‑litre four‑cylinder diesel. They are not interchangeable. This guide shows you exactly how to identify which engine is in your car, which chassis they fit, and why getting it right matters for replacement.
The OM642 and OM651 are both Mercedes diesel engines, but they are fundamentally different – the OM642 is a 3.0‑litre V6, while the OM651 is a 2.1‑litre four‑cylinder. They are not interchangeable due to different engine mounts, ECU programming, and physical dimensions. In 2025, the OM651 generated 1,250 quote requests, and the OM642 generated 880 [EM‑VERIFIED].
If you're unsure which engine is in your Mercedes, this guide will walk you through every identification step.
This table compares the OM642 and OM651 side by side, showing displacement, configuration, chassis codes, key models, and 2025 enquiry volumes. Use it to see which engine fits your car.
| Engine | Type | Displacement | Chassis Codes | Key Models | EM Requests 2025 |
|---|---|---|---|---|---|
| OM642 | Diesel V6 | 3.0 L (2987 cc) | W211, W212, W164, W166, X204, W251, W906 | E‑Class, ML/GLE, GLK, Sprinter, R‑Class | 880 [EM‑VERIFIED] |
| OM651 | Diesel I4 | 2.1 L (2143 cc) | W204, W205, W212, W213, X253, W447, W906, W907 | C‑Class, E‑Class, GLC, Sprinter, V‑Class | 1,250 [EM‑VERIFIED] |
While both engines appear in some of the same models (e.g., E‑Class W212, Sprinter W906), they are not cross‑compatible. The OM642 is physically larger, has a different bellhousing pattern, and requires specific engine mounts and exhaust routing. For a complete overview of all Mercedes fitments, see the full fitment matrix.
Use these three reliable methods to confirm whether you have an OM642 or OM651.
Open the bonnet and locate the sticker on the timing cover or camshaft cover. For an OM642, you'll see OM642 (e.g., OM642 DE30 LA). For the OM651, it will be OM651 (e.g., OM651 DE22 LA). This is the most direct method.
The 8th character of your VIN often indicates the engine family. For OM651, it's commonly a "2" or "9" (depending on variant); for OM642, it may be "6" or "7". Use our free Mercedes VIN decoder for a full breakdown, including the exact engine code.
The registration document lists the engine number, which typically starts with the engine code (e.g., 642... or 651...). Cross‑reference with our Engine Code Guide.
If you cannot locate the code, a vetted Mercedes specialist can identify it during a service.
Installing the wrong engine is one of the most expensive mistakes you can make. Here's why getting the code right is critical:
Once you've confirmed the engine, you can check replacement costs and use the repair vs. replace calculator to make an informed decision.
Even experienced owners sometimes confuse these two engines. Avoid these common mistakes:
If you are still unsure, use the full fitment matrix or contact a specialist.
The easiest way is to check the 8th character of your VIN. Use our VIN decoder to get a full breakdown. You can also look at the V5C logbook – the engine number will start with "642" or "651".
The OM651 is generally more reliable in terms of oil cooler seal issues (the OM642 is notorious for oil cooler seal leaks in the V of the engine). However, the OM651 can suffer from timing chain tensioner wear and injector seal failure (Black Death). Both have their weaknesses. For detailed reliability verdicts, see the OM642 Knowledge Centre and OM651 Knowledge Centre.
No – the engine mounts, ECU, wiring, exhaust, and cooling systems are all different. It is not a practical DIY swap and would cost more than simply replacing with the correct engine. Always replace like‑for‑like.
Supply‑only costs: OM642 reconditioned is around £3,200–£5,500, while OM651 is £2,800–£4,800 [EM‑QUOTE]. Labour for R&R is typically 10–14 hours for both, adding £650–£910 at independent rates. See the replacement cost guide for full details.
If the cost of a replacement engine (supply + fitting) exceeds approximately 70% of your car's current market value, it may be more economical to scrap the car. For an older OM642 in a W211 E‑Class, the threshold is often reached; for a newer OM651 in a W205 C‑Class, replacement is usually viable. See the repair/replace/scrap framework for worked examples.
Check the engine number on the block or the VIN decoder. The power output is often indicated by the suffix in the engine code (e.g., OM642 DE30 LA with different calibration codes). Your specialist can also read the ECU data to confirm the exact variant.
Knowing whether you have an OM642 or OM651 is the first and most important step before any engine work. Use the methods above – engine code label, VIN decoder, or V5C – to confirm which diesel you have. Then cross‑check the fitment matrix to ensure the engine matches your chassis.
Once identified, you can assess the repair or replacement cost. If your engine has failed, compare the reconditioned cost against your vehicle's value using the repair vs. replace calculator. Always use a vetted Mercedes specialist for the work.
Check Your Engine with Registration LookupReady to identify your engine or get a quote? Compare prices from vetted Mercedes specialists.