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Powertrain fault code

P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

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Not urgent, but fuel use goes up and the heater may blow cooler. Fix it at your next service.

What P0128 means

The engine is taking too long to reach normal operating temperature.

Common causes

  • Thermostat stuck open (most common)
  • Faulty coolant temperature sensor
  • Low coolant level

What to check first

  1. Check the coolant level when the engine is cold.
  2. Watch the live coolant temperature on a scan during a drive.
  3. Ask a workshop to test the thermostat.

P0128 on popular makes

P0128 on a Chevrolet

  • A thermostat stuck partly open is the most common cause: the engine takes too long to warm up.
  • Low coolant or a faulty coolant temperature sensor can also set it; check the level first.

Get P0128 explained for your car: $2.99

  • P0128 explained in full for your make and model: meaning, how urgent, likely causes and what to check, in order.
  • A short personal note from us within 24 hours, or your money back.
  • Your own private report page you can save as a PDF or show your mechanic.

Read your car's codes yourself, free

Plug a cheap ELM327 OBD-II adapter into the port under the dashboard and open the scan tool: Chrome or Edge on a computer with a USB adapter, or Chrome on Android with a Bluetooth LE adapter. It reads and clears codes and shows emissions readiness. It does not work on iPhone.

Open the free scan tool

Need help with P0128?

Tell us your car and what it is doing with P0128. We reply by email with what to check first, in plain words. Free.

Related codes

P0125

General guidance only, not a diagnosis. A qualified mechanic should confirm the cause before you replace parts.