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P0128

Coolant temperature below thermostat regulating temperature

The engine is taking too long to reach the expected operating temperature.

What this code means

The engine is taking too long to reach the expected operating temperature.

Possible causes

  • Thermostat stuck open
  • Low engine coolant level
  • Faulty engine coolant temperature sensor
  • Coolant temperature sensor wiring issue

Common symptoms

  • Check engine light
  • Temperature gauge stays low
  • Cabin heater may be weak
  • Reduced fuel economy

Checks to make

  1. Check coolant level when the engine is cold
  2. Check for visible coolant leaks
  3. Review coolant-temperature live data
  4. Inspect thermostat operation
  5. Check for related coolant temperature sensor codes

Safety

  • Never open a hot pressurised cooling system. Allow the engine to cool fully before checking coolant.

Model notes

The most common cause is often a thermostat that is stuck open, but diagnosis should confirm the fault.