The OM646 bridged the gap between the OM611 and OM651. It is found in the C-Class W203/W204, E-Class W211, Sprinter, and Vito. Second-generation Bosch common-rail makes it more reliable than the OM611 but still prone to age-related injector wear.
Rough idle with injector knock: The OM646 develops a distinctive knock at idle when injector nozzles are worn.
Black smoke and poor acceleration: Over-fuelling from worn nozzles causes black smoke and sluggish response.
Starting difficulties: Age-degraded injectors struggle to atomise fuel at cranking speeds.
Fuel smell from engine bay: External leaks at injector seals produce a noticeable diesel smell.
Why OM646 injectors fail
fuel contamination
high-pressure wear cycles
carbon buildup
poor fuel quality
internal valve wear
How to diagnose OM646 injector faults
Read fault codes: Connect an OBD-II scanner and note any injector-related codes (P0201–P0204, P2146, P2149). This identifies which cylinder is affected.
Perform a leak-off test: Disconnect the fuel return pipes and route into measuring containers. Run the engine at idle for 60 seconds. Compare volumes per cylinder against manufacturer spec.
Listen for injector tick: Use a mechanic's stethoscope on each injector body. A dead or irregular tick indicates a stuck or failing solenoid.
Check fuel rail pressure: Monitor live fuel rail pressure data via diagnostic tool. Low pressure can mimic injector symptoms and may indicate a fuel pump issue instead.
Inspect for external leaks: Check around injector seals for signs of fuel or combustion gas leakage. Black carbon deposits indicate a blown seal.
Cylinder balance test: Use diagnostic software to run a cylinder contribution test. This electronically disables each injector in turn to measure its effect on engine speed.
Fault codes linked to OM646 injector issues
P0201-P0204 – Injector circuit malfunction
P0087 – Fuel rail pressure too low
P0093 – Fuel system leak detected
OM646 repair advice
Early OM646 injectors are plug-and-play. Later variants may require basic coding. Check glow plug condition simultaneously.
Early OM646 engines do not require coding. Later 2007+ models may require basic coding via Star Diagnostic.
How much does OM646 injector replacement cost?
A set of four remanufactured injectors costs £350-£600. Labour is 2-3 hours.
Is the OM646 reliable?
Yes, the OM646 is considered reliable. Most injector issues appear after 120,000+ miles.
How much does it cost to replace injectors?
Replacement typically costs £150–£350 per injector depending on brand and engine. A full set of four with professional fitting usually costs £600–£1,200.
Can I drive with a faulty injector?
Driving with a faulty injector can cause engine damage, increased fuel consumption, DPF contamination, and oil dilution. We recommend diagnosis and replacement as soon as possible.
Do new injectors need coding?
Some vehicles require injector coding after replacement using manufacturer-specific diagnostic tools. Many Bosch solenoid units are plug and play. Check your engine guide for specific requirements.
UK's Leading Diesel Injector Specialists
Find the Exact Fuel Injector by Vehicle Registration
UK diesel injector specialists supplying OEM tested and calibrated remanufactured injectors. Trusted by garages and mechanics nationwide with 12-month warranty on every part.
