What it means
Coolant temperature dropped below the diagnostic monitoring threshold after warm-up. Often thermostat stuck open or stuck-open coolant heater bypass.
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 watches coolant temperature during a diagnostic monitoring period after warm-up. This code sets when the measured temperature falls back below that monitoring threshold, which can indicate that the engine is cooling unexpectedly or that the temperature reading is inaccurate.
Common causes
Most likely first.
- Thermostat stuck open
- Coolant temperature sensor reading inaccurately
- Cooling-system issue causing unexpected temperature drop
Symptoms
- Check-engine light
- Coolant temperature gauge drops after warming up
- Cabin heat may weaken while driving
How to diagnose it
- Check freeze-frame data and coolant-temperature live data to see when the temperature fell and whether the engine had reached normal operating temperature.
- Compare the coolant-temperature reading from a cold engine with ambient conditions and look for sudden or implausible changes as the engine warms.
- Inspect coolant level and look for leaks or other visible cooling-system problems, checking only when the engine is cool.
- Observe whether temperature drops during steady driving and whether cabin heat weakens, clues that the thermostat may be stuck open.
- If the temperature pattern does not match the engine's behavior, test the coolant-temperature sensor and its wiring before replacing parts.
Common fixes
- Repair a coolant leak or correct a low coolant level after locating its cause.
- Replace a thermostat that is confirmed to remain open.
- Repair damaged wiring or connections to the coolant-temperature sensor.
- Replace a coolant-temperature sensor that gives inaccurate readings.
Can I keep driving?
If the temperature gauge stays in its normal range and coolant is not low, a short drive may be possible, but monitor the gauge closely. Stop if the engine overheats or coolant warning appears; continued operation with abnormal temperature can cause engine damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.