P0125 — Insufficient Coolant Temperature for Closed Loop Fuel Control

After starting, the controller does not see the coolant temperature required for closed-loop fuel control within the allowed time. Distinguish genuinely insufficient warm-up from an inaccurate ECT input. This code does not automatically identify a failed oxygen sensor.

Affected system: Powertrain

Symptoms

• Slow warm-up and weak cabin heat if the engine remains cold.
• Higher consumption or engine warning light; other noticeable symptoms may be absent.

Possible causes

• Leaking/stuck-open thermostat or low coolant level.
• Incorrect ECT sensor/circuit input; inadequate warm-up conditions.

What to check

• With the engine cold, check coolant level and leaks; save ambient temperature and elapsed run time from freeze-frame data.
• Compare cold ECT with ambient temperature, then log warm-up together with fuel-loop status.
• Compare actual engine temperature with ECT and the thermostat range specified for that engine, using the OEM method.
• If the engine stays cold, test the thermostat; if only ECT is wrong, investigate the sensor, connector and circuit.
• After repair, clear codes and recheck warm-up and closed-loop entry under the original recorded conditions, following the manufacturer’s procedure.

Also written as: P0125-00, P012500, PO125 (some scan tools add the fault-type byte, e.g. -00).