P0117 — Engine Coolant Temperature Sensor Circuit Low
Moderate severityWhat it means
P0117 is stored when the engine control unit sees a fault involving the engine coolant temperature (ECT) sensor signal. The ECT sensor reports engine temperature, which drives cold-start enrichment, the cooling fan and the temperature gauge. A “low” reading means the signal is stuck near the bottom of its range, which usually points to a short to ground or an internally failed sensor.
Common symptoms
- The live reading sits abnormally low or at zero
- Hard cold starts or rough warm-up
- Cooling fan running constantly or not at all
- Poor fuel economy
Likely causes
- Signal wire shorted to ground
- Internally failed sensor pulling the signal low
- Faulty ECT sensor
- Low coolant or a wiring/connector fault
What to check first
- Compare the live signal against a known-good range and check for a short to ground
- Watch the coolant temperature climb from cold to about 90°C in live data
- Compare ECT and IAT on a cold engine — they should match
Can you keep driving?
Often drivable, but wrong temperature data wastes fuel and can cause overheating or fan problems — fix it soon.
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