P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control

What it means

The engine has run long enough that closed-loop fuel control should be active, but the coolant temperature has not risen high enough. Often caused by a stuck-open thermostat or a failed coolant temperature sensor.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The engine control module uses coolant temperature to decide when to switch from warm-up fueling to closed-loop fuel control. P0125 sets when the engine has run under qualifying conditions but the reported coolant temperature has not reached the level needed for that transition.

Common causes

Most likely first.

1. Thermostat stuck open
2. Coolant temp sensor reading low

Symptoms

• Check engine light on
• Longer engine warm-up
• Weak cabin heat
• Reduced fuel economy

How to diagnose it

1. Review freeze-frame data and warm-up live data to see whether coolant temperature rises as expected after a cold start.
2. Compare the coolant temperature reading before starting with ambient temperature to check for a sensor reading biased low.
3. Observe whether the engine takes unusually long to warm and whether cabin heat remains weak, which can point toward a thermostat stuck open.
4. Inspect the coolant temperature sensor connector and wiring for corrosion, damage, or poor contact.
5. Test the thermostat and sensor circuit against the manufacturer's specification before replacing either part.

Common fixes

• Replace a thermostat confirmed stuck open.
• Repair a poor connection or damaged coolant temperature sensor wire.
• Replace the coolant temperature sensor if its reading is inaccurate.

Can I keep driving?

If the temperature gauge stays normal, cautious driving is usually possible, but expect weak cabin heat and increased fuel use. Stop if the engine overheats or coolant is leaking.