ML420 CDI 4.0 V8 (OM629)
2006–2010 · diesel · 3996cc · 306hp
Engine codes: OM629
Reliability, common problems and owner reviews for the Mercedes Benz M-Class (ML) ML420 CDI 4.0 V8 (OM629). Check this before you buy used.
The 4.0 V8 CDI (OM629) is a magnificently torquey, smooth diesel — but it's a rare, complex twin-turbo V8 in an SUV: turbo, injection and emissions repairs are costly, parts are scarce, and few specialists know it well. Mechanically robust when maintained, but ownership cost and parts availability are the real risks. Only worth it with full history and deep pockets; the 3.0 V6 CDI does 90% of the job for far less.
Known Issues
V8 twin-turbo, injection and emissions repairs are expensive; parts scarce.
Fix / Workaround: Specialist diagnosis; budget heavily.
Repair cost: €600–€3000
Typically appears after: 150,000 km
Mileage Thresholds
After 180,000 km: Expensive, niche V8 diesel.
Pre-Purchase Checklist
- ☐Full history + specialist access
- ☐Parts scarcity
- ☐3.0 V6 CDI is the smarter choice
Frequently asked questions
Is the Mercedes Benz M-Class (ML) ML420 CDI 4.0 V8 (OM629) reliable?
Expensive — The 4.0 V8 CDI (OM629) is a magnificently torquey, smooth diesel — but it's a rare, complex twin-turbo V8 in an SUV: turbo, injection and emissions repairs are costly, parts are scarce, and few specialists know it well. Mechanically robust when maintained, but ownership cost and parts availability are the real risks. Only worth it with full history and deep pockets; the 3.0 V6 CDI does 90% of the job for far less.
What are the common problems and reviews for the Mercedes Benz M-Class (ML) ML420 CDI 4.0 V8 (OM629)?
The most commonly reported problems: Twin-turbo / injection cost.
Is a used Mercedes Benz M-Class (ML) ML420 CDI 4.0 V8 (OM629) worth buying?
Won't fail catastrophically, but when things do go wrong the repairs cost a lot. Budget accordingly.