Thermostat codes

How its codes differ

• P0128 is the warm-up test: coolant did not reach the expected temperature in the expected time. A thermostat stuck open is the usual cause, though a coolant temperature sensor that reads low can also set it.
• A thermostat stuck closed causes the opposite: overheating. That is a stop-driving condition whatever code is stored.
• P0125 means the engine took too long to get warm enough for closed-loop fuel control; a thermostat stuck open is a common cause.

Codes

• P0128

  Engine coolant stays below thermostat temperature

  Severity: Monitor

• P0125

  Coolant too cold for closed-loop fuel control

  Severity: Monitor

Signs it's failing

• Engine never reaches normal operating temperature (stuck open)
• Check engine light with P0128
• Overheating (stuck closed)

How to test it

1. Watch the temperature gauge or scan-tool coolant temperature on a drive: a gauge that never reaches its normal position fits a thermostat stuck open, because the engine cannot reach operating temperature. (Low risk)

2. Replacing a thermostat means draining coolant; have a shop do it if you are not experienced. (Best left to a shop)

   Warning: Never open the radiator or coolant reservoir cap while the engine is hot: pressurised coolant can spray and scald.

After the repair

• Check that the temperature gauge reaches its normal position and stays there.
• Check coolant level once the engine is fully cold.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.