CKFCar Known Faults
MinorAffected: 2012-201760,000–120,000 miles

Ford Fiesta Mk7 1.0 EcoBoost 100bhp — 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.

Cooling diagram
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

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

Estimated Repair Costs

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

Other Faults on This Variant

Timing Chain Stretch and Tensioner Failure

Critical
2012-201540,000–100,000 miles£600–£1200

Pre-2016 Ford 1.0 EcoBoost engines suffer from premature timing chain stretch. The plastic tensioner can fail, causing catastrophic engine damage with little warning.

Coolant Loss from Cylinder Head

Severe
2012-201860,000–120,000 miles£500–£900

The 1.0 EcoBoost has an integrated exhaust manifold cast into the cylinder head. This design causes head gasket stress and coolant loss, sometimes causing overheating.

Thermostat Failure and Overheating Risk

Moderate
2012-201850,000–100,000 miles£80–£180

The plastic thermostat housing on the 1.0 EcoBoost is prone to cracking. A stuck-open thermostat causes the engine to run cold and burn more fuel; a stuck-closed thermostat causes rapid overheating.

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