MinorAffected: 2005-201160,000–120,000 miles

Toyota Yaris Mk2 1.3 VVT-i 87bhp — 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

Water Pump Leak and Bearing Failure

Moderate

2005-201160,000–100,000 miles£150–£250

While incredibly reliable, the 1.3 VVT-i engine's most common weak point is the water pump, which develops a leak and noisy bearing.

MMT Semi-Automatic Gearbox Actuator Failure

Critical

2005-201150,000–90,000 miles£500–£1500

Models fitted with the Multi-Mode Transmission (MMT) — an automated manual — suffer from severe actuator wear, causing the car to drop into neutral while driving.

Air Conditioning Condenser Leak (Stone Damage)

Minor

2005-201140,000–120,000 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.

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