P0128: OBD-II code P0128 means the engine is not reaching operating temperature within the expected time — almost always a stuck-open thermostat.

Symptoms

• The check engine light illuminates after the engine has been running for a while, especially in cold weather.
• The temperature gauge never rises to normal operating range, or takes far longer than usual to warm up.
• Cabin heat output is weak or slow because the coolant never reaches the temperature the heater core needs.

Common causes

• A stuck-open thermostat that lets coolant circulate through the radiator before the engine reaches operating temperature.
• A faulty engine coolant temperature (ECT) sensor reading falsely low values to the PCM.
• Cold ambient temperatures combined with extended idle or short trips — a condition-based fault, not always a hardware issue.

What to do?

Verify the ECT sensor reads accurately against an infrared thermometer on the thermostat housing. If the sensor is good, replace the thermostat with an OEM-spec part (aftermarket thermostats often fail this test). Refill and bleed the coolant properly. Confirm the engine reaches about 195 °F within 10-15 minutes of driving.