Engine
What does the engine thermostat do and what does P0128 mean?
The thermostat is the valve that lets coolant through to the radiator once the engine is hot, so the engine warms up quickly and stays at its temperature. If it sticks open, the engine takes long to warm up and the control unit stores P0128; if it sticks closed, the engine overheats, and that does mean stopping.
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What happens if it sticks open?
The engine takes a long time to reach its temperature, or never does in winter. You notice weak heat, a low temperature gauge and slightly higher fuel use.
The control unit compares how long it takes to warm up with what it expects and stores P0128. It is not urgent, but worth fixing: a cold engine uses more fuel and wears more.
And if it sticks closed?
Coolant does not reach the radiator and the temperature climbs fast. If you see the temperature light or the gauge in the red, stop as soon as it is safe and switch off: carrying on can damage the cylinder head.
How is it checked?
In live data you watch how coolant temperature rises from cold. The temperature sensor (P0115 to P0118) has to be ruled out, since it can give a false reading.
Some cars have thermostats controlled by the engine control unit, with their own codes and test procedures.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0128Coolant thermostat: coolant temperature below the thermostat regulating temperature
- P0115Engine coolant temperature sensor 1, circuit: malfunction
- P0117Engine coolant temperature sensor 1, circuit: signal low
- P0118Engine coolant temperature sensor 1, circuit: signal high
- P0217Engine coolant: over temperature
Related symptoms
- Engine temperature warning light on: what to do
- Car uses more fuel than normal: causes and what to check
- White smoke from the exhaust: what it means and what to do
Learn more
See what your car is saying
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Frequently asked questions
Can I drive with P0128?
Usually yes, if the temperature does not rise above normal. Fix it soon because of fuel use and wear.
Can low coolant cause P0128?
It can affect the sensor reading. Check the level with the engine cold.
Should I replace the sensor or the thermostat?
It depends on which one fails: live data and a test make it clear.