What does OBD code P0128 mean?
P0128 means the engine coolant temperature is below the thermostat regulating temperature. In plain English, the engine is running cooler than it should be. This almost always means the thermostat is stuck open — coolant circulates continuously rather than building up to operating temperature. A cold-running engine uses more fuel, produces more emissions, and may not provide adequate cabin heat.
Likely causes
Thermostat stuck open
The most common cause — coolant temperature never reaches normal operating temperature on the temperature gauge
Faulty coolant temperature sensor
A sensor reporting lower than actual temperature triggers P0128 without a real thermostat fault
Wrong thermostat fitted
A thermostat with a lower opening temperature than specified — sometimes happens after DIY repairs
Repairs you may need
Typical cost range: £80–£220 — Thermostat replacement: £80–220 including labour. Coolant temperature sensor: £60–150.
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