Engine Code Guide – Mercedes Fitment & Identification

Q: What does "OM" stand for in Mercedes engine codes?

"OM" stands for Öl-Motor, which is German for oil engine – i.e., diesel. "M" stands for Motor and is used for petrol engines. This naming convention has been in place for decades.

Q: How much does it cost to replace a Mercedes engine?

Costs vary widely. A reconditioned OM651 supply-only is around £2,800–£4,800, while an OM642 V6 is £3,200–£5,500, and an M276 petrol V6 is £3,800–£6,500 [EM-QUOTE]. See the replacement cost guide for a full breakdown.

The Short Answer

Your Mercedes engine code is a combination of letters and numbers that uniquely identifies the engine family, displacement, and configuration. It is stamped on the engine block, encoded in the 8th character of your VIN, and listed on the V5C logbook. Common codes include OM651 (2.1 diesel), OM642 (3.0 V6 diesel), M276 (3.0 V6 petrol), and M177 (4.0 V8 biturbo).

Where to find it: Engine block label, timing cover sticker, VIN (8th digit), or registration document.
Why it matters: The wrong engine won't fit, won't talk to the ECU, and can cause MOT or insurance issues.
Most requested engines in 2025: OM651 (1,250 enquiries), OM642 (880), OM654 (520), M276 (390) [EM-VERIFIED].
Next step: Use our VIN decoder or full fitment matrix to confirm compatibility.

If you already have a failed engine, the cost guide will help you understand the financial impact.

What Is a Mercedes Engine Code?

A Mercedes engine code is a short alphanumeric identifier that tells you exactly which engine is fitted to your car. It reveals the fuel type, displacement, cylinder configuration, and generation. For example, OM651 means a 2.1‑litre four‑cylinder diesel (the "OM" stands for Öl-Motor – diesel in German), while M276 is a 3.0‑litre V6 petrol.

Every chassis – from the A‑Class (W176) to the S‑Class (W222) – is designed for a specific set of engine codes. Installing an engine that isn't listed for your chassis will not fit mechanically, will not communicate with the ECU, and will fail an MOT. This is why knowing your code is the first and most important step before any replacement or swap.

In our 2025 enquiry data, the top requested Mercedes engines were OM651 (1,250 requests), OM642 (880), OM654 (520), and M276 (390) [EM-VERIFIED]. These represent the most common fitments across the C‑Class, E‑Class, GLC, and GLE ranges.

Where to Find Your Mercedes Engine Code

There are four reliable ways to locate your engine code. Use the one that suits your situation best.

On the engine block itself.
Most Mercedes engines have a stamped metal plate or a machined pad on the cylinder block. For OM651, it's on the left side near the oil filter housing. For M276, it's on the front face behind the intake manifold. For detailed diagrams and photos, see our engine number location guide.
On the timing cover or camshaft cover label.
Many modern engines have a sticker on the timing cover with the engine code and other details. This is often the quickest spot to check without any disassembly.
From the VIN (Vehicle Identification Number).
The 8th character of your VIN indicates the engine family. For example, a VIN with the 8th character "9" often points to an OM651 diesel, while "6" may indicate an M276 petrol. Use our free Mercedes VIN decoder for a full breakdown.
On your V5C registration document.
The logbook includes the engine number, which can be cross‑referenced with our fitment matrix to confirm the code.

If you cannot find the code using these methods, a vetted Mercedes specialist can locate it for you during a service or inspection.

How to Decode Your Mercedes Engine Code

Mercedes engine codes follow a consistent pattern. The first two letters indicate the fuel type: OM for diesel (Öl-Motor) and M for petrol (Motor). The following numbers usually denote displacement (in litres x 10) and configuration, though the exact meaning varies by generation.

Code Prefix	Fuel Type	Typical Displacement	Common Models
OM	Diesel	2.1 – 3.0	C‑Class, E‑Class, GLC, Sprinter
M	Petrol	1.6 – 6.0	All models, including AMG
OM65x	Diesel (modern 4‑cyl)	2.0	W213, X253, V167
M27x	Petrol (V6)	3.0 – 3.5	E‑Class, CLS, GLE
M17x	Petrol (V8 biturbo)	4.0	AMG GT, C63, E63

For a full breakdown of every engine code and its specifications, see our Engine Knowledge Centre pages.

Common Mercedes Engine Families and Their Fitments

The table below summarises the most frequently encountered engine families and the chassis they were fitted to. For a complete list, visit the full fitment matrix.

Engine Code	Type	Chassis Codes	Key Models
OM651	2.1 Diesel	W204, W205, W212, W213, X253, W447, W906	C‑Class, E‑Class, GLC, Sprinter
OM654	2.0 Diesel	W205, W206, W213, W214, X253, X254, V167	C‑Class, E‑Class, GLC, GLE
OM642	3.0 V6 Diesel	W211, W212, W164, W166, X204, W251, W906	E‑Class, ML/GLE, GLK, Sprinter
M276	3.0 V6 Petrol	W212, W213, W166, C218, X166	E‑Class, CLS, GLE, GLS
M274	2.0 Petrol	W205, W213, X253	C‑Class, E‑Class, GLC
M177	4.0 V8 Biturbo	W205, W213, W222, X166, V167	C63, E63, S‑Class, GLE AMG

Always confirm the exact variant (e.g., OM651 DE22 LA vs. OM651 DE22 LA EVO) because ECU programming and ancillaries differ.

Why Correct Engine Identification Matters

Installing the wrong engine is one of the most expensive mistakes you can make. Here's why getting the code right is non‑negotiable:

ECU compatibility: The engine control unit is programmed for a specific engine family, fuel system, and emissions setup. A mismatched engine will not run correctly or may not start at all.
Physical fitment: Engine mounts, exhaust manifolds, cooling pipes, and wiring looms differ between engine families. Even within the same family, variant‑specific parts can prevent a direct swap.
Legal compliance: Fitting an engine not originally homologated for your chassis can cause MOT failure, insurance invalidation, or DVLA issues.
Warranty and specialist support: Most vetted specialists will refuse to work on a mismatched combination. Always check the fitment matrix and consult a specialist before purchasing.

Once you know your engine code, you can use our Engine Compatibility Checker to see which other chassis share the same unit.

Common Misidentifications and Mistakes

Even experienced owners sometimes confuse similar codes. Avoid these pitfalls:

OM651 vs. OM654: Both are 2.0‑ish diesels, but OM651 is the older 2.1‑litre unit (2008–2019), while OM654 is the newer 2.0‑litre modular engine (2016+). They are not interchangeable. Check the VIN 8th character or the engine block label.
M272 vs. M276: Both are V6 petrols, but M272 is a 3.5‑litre (2004–2011) with a balance‑shaft issue, while M276 is a 3.0‑litre (2011+) with a different timing chain design. They have different engine mounts and ECU pinouts.
M157 vs. M177: Both are biturbo V8s. M157 is a 5.5‑litre (2012–2018) found in E63, S63, and CLS63. M177 is a 4.0‑litre (2015+) used in C63, E63, and AMG GT. They share some architecture but are not physically identical; replacement parts are not cross‑compatible.
Om642 vs. OM651: The V6 diesel (OM642) is physically larger and has a different bellhousing pattern than the four‑cylinder OM651. Fitting a 651 into a V6 chassis would require custom mounts and adapters – not a practical DIY job.

If you are unsure, use the OM vs. M‑series identification guide or contact a specialist.

Frequently Asked Questions

How do I find the engine code on my Mercedes without lifting the car?

The easiest method 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 registration document, which lists the engine number. The engine code is usually the first part of that number.

What does "OM" stand for in Mercedes engine codes?

"OM" stands for Öl-Motor, which is German for oil engine – i.e., diesel. "M" stands for Motor and is used for petrol engines. This naming convention has been in place for decades.

Can I fit an engine from a different Mercedes model into my car?

Only if the engine code is listed for your chassis in the fitment matrix. For example, an OM651 from a W204 C‑Class will also fit a W212 E‑Class or X253 GLC, but you must check the specific variant and ECU compatibility. Always consult a specialist before buying.

What is the most common Mercedes engine in the UK?

Based on our 2025 enquiry data, the OM651 2.1 diesel is the most requested, with 1,250 quotes [EM-VERIFIED]. It appears in the C‑Class, E‑Class, GLC, and Sprinter. The OM642 V6 diesel follows with 880 requests.

How much does it cost to replace a Mercedes engine?

Costs vary widely. A reconditioned OM651 supply‑only is around £2,800–£4,800, while an OM642 V6 is £3,200–£5,500, and an M276 petrol V6 is £3,800–£6,500 [EM-QUOTE]. See the replacement cost guide for a full breakdown.

What does the 70% scrap threshold mean for Mercedes?

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. This threshold is a genuine guideline based on our proprietary data. For worked examples, see the repair/replace/scrap framework.

Our Verdict: Start Here Before Any Engine Work

✅ Essential

Knowing your engine code is the single most important step before ordering a replacement, checking compatibility, or even diagnosing a fault. Use this guide to locate your code, decode it, and confirm that it matches your chassis. Then use the VIN decoder and fitment matrix to double‑check.

Once you have confirmed the correct engine, the next step is to understand the cost. Visit the replacement cost guide and use the repair vs. replace calculator to make a financially sound decision.

Check Your Engine with Registration Lookup