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CoolingSeverity Low: usually safe to drive

P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What it means

The engine doesn't reach (or keep) its normal operating temperature within the time the ECU expects. The reading is plausible, so the sensor is usually fine and the cooling system is to blame.

Symptoms

  • Temperature gauge stays low, especially on the motorway
  • Weak heating in winter
  • Higher fuel consumption

Common causes

  • Thermostat stuck open
  • Wrong thermostat fitted
  • Coolant temperature sensor reading low
  • Radiator fan running all the time

What to check first

  1. Watch coolant temperature on a steady road drive
  2. Check whether the top radiator hose warms up within the first few minutes from cold (it shouldn't)
  3. Compare the sensor reading with an IR thermometer
Can I drive with P0128? Usually yes. P0128 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.
Practise it: Thermostat stuck open in the engine simulator shows this fault in live data, tests and on the scope.Open in the simulator

Generic OBD-II meaning. Manufacturer-specific meanings and checks can differ; always check the service information for the car.

How to test the parts behind P0128

Coolant temperature sensor (ECT)Wiring, typical values, multimeter and scope tests

Known engine problems that set P0128

1.6 THP/Prince: Coolant leaks: water pump and thermostat housingMini, BMW, Peugeot, Citroën, DS · typically 60,000–150,000 km

Cases with P0128 (1)

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Particulate filter light, limp mode and oil level risingTraining case · 2015 Dacia Duster · 1.5 dCi 110 (K9K) · fixed: Thermostat stuck partly open: the engine never got warm enough to finish a regen

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P0128: questions and answers

What does P0128 mean?

P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). The engine doesn't reach (or keep) its normal operating temperature within the time the ECU expects. The reading is plausible, so the sensor is usually fine and the cooling system is to blame.

What are the symptoms of P0128?

Temperature gauge stays low, especially on the motorway. Weak heating in winter. Higher fuel consumption.

What causes P0128?

Thermostat stuck open. Wrong thermostat fitted. Coolant temperature sensor reading low. Radiator fan running all the time.

How do you diagnose P0128?

1. Watch coolant temperature on a steady road drive 2. Check whether the top radiator hose warms up within the first few minutes from cold (it shouldn't) 3. Compare the sensor reading with an IR thermometer

Can I drive with P0128?

Usually yes. P0128 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.