Cooling

P0125 — Coolant Temperature Too Low for Closed Loop Fuel Control

Severity: low Safe to drive

What P0125 means

After a cold start, the engine computer expects coolant to reach a set temperature within a certain time so it can switch to closed loop fuel control, using the oxygen sensor feedback. If the engine warms too slowly, or the coolant temperature sensor reads too low, the computer sets P0125 and keeps running on a richer, fallback fuel mixture.

Symptoms

  • Check Engine light on
  • Worse fuel economy
  • Weak cabin heater
  • Temperature gauge stays low or rises slowly
  • Engine takes a long time to warm up
  • Possible fuel smell or black exhaust smoke

Possible causes

  1. Thermostat stuck open
  2. Low coolant level or a coolant leak
  3. Faulty engine coolant temperature (ECT) sensor
  4. Damaged wiring or connector at the ECT sensor
  5. Air trapped in the cooling system
  6. Very cold ambient temperatures
  7. Cooling fan running constantly

How to diagnose

  1. Scan for codes and freeze frame data, and look for related coolant temperature or oxygen sensor codes
  2. Check coolant level and condition with the engine cold, and look for leaks
  3. Compare the scan tool ECT reading with actual temperature measured by an infrared thermometer on the hose
  4. Warm the engine and see whether the upper radiator hose heats up at the right time, which shows the thermostat is opening properly
  5. Inspect the ECT sensor wiring and connector for breaks, corrosion or shorts to ground
  6. Measure the ECT sensor resistance and compare it to the spec for your vehicle
  7. Check whether the radiator fan is running all the time
  8. Make sure the cooling system has been bled of air

Common fixes

  • Replace the thermostat
  • Top up coolant and repair any leak
  • Replace the ECT sensor
  • Repair the sensor wiring or connector
  • Bleed air from the cooling system

P0125 on different brands

Ford · manuals
Ford uses P0125:00 in its gasoline engine monitor strategy to mean insufficient coolant temperature for closed loop fuel control; it describes the monitored condition, not a confirmed failed part.

Questions and answers

Can I drive with a P0125 code?

Usually yes, but don't wait too long. The engine runs rich, which hurts fuel economy, and a cause like a bad thermostat can get worse.

What most often causes P0125?

A thermostat stuck open is the most common cause. Low coolant and a faulty ECT sensor or its wiring come next.

Can P0125 appear just because it's cold outside?

Yes, in very cold weather and on short trips the engine may not warm up in time. If the code returns in mild weather, look for a real fault.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.