🌡️ Coolant Temperature Codes
TL;DR
Coolant-temperature codes (P0116–P0118 sensor, P0125/P0128 engine not warming up) usually mean a stuck-open thermostat or a faulty coolant temperature sensor. Severity: low–medium. Symptoms: slow warm-up, weak heater, poor economy. Check the thermostat and the ECT sensor first; verify the coolant level too.
Codes in this family
Shared causes
- A stuck-open thermostat (engine runs too cold — P0128)
- A faulty coolant temperature (ECT) sensor
- Low coolant level
- Wiring or connector fault to the sensor
- A stuck-closed thermostat (overheating — the dangerous case)
Parts commonly involved
Related symptoms
Frequently asked questions
Is a P0128 always the thermostat?
Not always, but a stuck-open thermostat is the most common cause of P0128 (coolant temp below the regulating temperature). A faulty coolant temperature sensor or low coolant can also cause it, so verify before replacing.
What is the difference between the ECT sensor and the thermostat?
The thermostat is a mechanical valve that controls coolant flow; the coolant temperature sensor is an electronic probe that reports temperature to the computer. Both can cause these codes, so check which is at fault.
Can a coolant temperature fault hurt fuel economy?
Yes. If the computer thinks the engine is cold it keeps adding extra fuel, hurting economy and sometimes causing black smoke and rough cold running.