Error code

OBD2 P0128

OBD2P0128
Diagnostic Trouble Code· Standard OBD-II fault code read from the vehicle's onboard diagnostics.

What it means

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).

Severity

moderateCan drive, but may experience poor fuel efficiency and increased emissions.

Summary

P0128 indicates that the coolant temperature is too low for the thermostat's regulating temperature, suggesting an issue with the cooling system or thermostat.

Symptoms

  • Engine running too cold
  • Coolant not reaching operating temperature

Common Causes

  • Improper coolant level/mixture
  • Faulty cooling fan
  • Faulty coolant temperature sensor (ECT)
  • Stuck open thermostat

Diagnosis

  1. Check coolant level and condition
  2. Test cooling fan operation
  3. Verify ECT sensor readings with a scan tool
  4. Measure radiator hose temperatures to test thermostat

Fixes

  • Replace faulty parts such as the thermostat, coolant temperature sensor, or cooling fan

FAQ

What should I do if my car has P0128?

Check your coolant level and condition first. Then test the cooling fan, ECT sensor, and thermostat to identify the faulty component.

Is it safe to drive with a P0128 code?

It is generally drivable but may result in poor fuel efficiency and increased emissions until fixed.

View source

Not affiliated with OBD2. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.

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