P0125: OBD-II code P0125 means the coolant is not reaching the temperature needed for closed-loop fuel control — the engine stays in open-loop enrichment for too long.

Symptoms

• Check engine light illuminates after extended cold-weather driving or a series of short trips.
• Fuel economy drops noticeably because the PCM keeps running rich open-loop fuel mixtures.
• Cabin heat is weak or slow, and the temperature gauge may not reach normal operating range.

Common causes

• A stuck-open thermostat letting coolant flow through the radiator before the engine warms up.
• A failed engine coolant temperature (ECT) sensor reading falsely cool to the PCM.
• Extreme cold weather combined with short trips — a condition-based fault, not always a hardware issue.

What to do?

Verify ECT reads accurately against an IR thermometer on the thermostat housing. Replace the thermostat with an OEM-spec part if it opens too early. Confirm the engine reaches about 195 °F within 10-15 minutes of highway driving.