MinorAffected: 2007-201460,000–120,000 miles

Mercedes-Benz C-Class W204 C220 CDI — Engine Thermostat Sticking Open

The engine coolant thermostat fails in the 'open' position. The engine never reaches its optimal operating temperature, killing fuel economy.

Severity

Minor

DIY Difficulty

Moderate

Est. Cost

£100–£250

OBD Codes

P0128

Component Diagram

Cooling

Typical layout and location for the Cooling assembly.

Reference Illustration

Symptoms

Temperature gauge drops when driving on the motorway
Heater blows lukewarm air in winter
MPG drops significantly
Engine takes a very long time to warm up

Root Cause

The wax element inside the thermostat degrades, or the return spring weakens, preventing the valve from closing off coolant flow to the radiator.

How To Fix

Scan for P0128 (Coolant Thermostat - Coolant Temp Below Regulating Temp).
Replace the thermostat housing unit and gasket.
Bleed the cooling system of air.
Cost: £100-250

Estimated Repair Costs

Repair Option	Est. Cost (GBP)
DIY (parts only)	£40–£100
Independent Garage	£100–£250
Main Dealership	£160–£400

Other Faults on This Variant

Delphi Piezo Injector Failure

Critical

2008-201230,000–100,000 miles0

Early OM651 engines (2008-2012) were fitted with Delphi piezoelectric injectors that failed so frequently it resulted in a massive global recall and class-action lawsuits.

Air Conditioning Condenser Leak (Stone Damage)

Minor

2007-201440,000–120,000 miles£250–£450

The air conditioning condenser is mounted at the very front of the cooling pack, making it highly susceptible to stone damage from the road, leading to refrigerant loss.

Front Anti-Roll Bar Drop Link Wear

Minor

2007-201430,000–80,000 miles£60–£120

The front anti-roll bar drop links wear out on heavily potholed UK roads, causing an annoying knocking sound at low speeds.

Disclaimer: Repair cost estimates are indicative and based on community data. Always get a quote from a qualified mechanic before proceeding with any repair.