Fault Tree: P0117 / P0118 Coolant Temp Sensor Circuit
These codes are usually an electrical problem (wiring, connector, sensor ground/reference), but always sanity-check the temperature reading first.
Quick triage & tools
- Scan first: read codes + freeze-frame. Note coolant temp, intake air temp, and ambient temp.
- KOEO sanity: key-on engine-off, ECT should be close to ambient after sitting.
- Have ready: scan tool with live data, multimeter, and (ideally) an IR thermometer.
Wiring help: Reference 5V & sensor grounds → · Wiring diagrams basics →
What you need (minimal)
- Live data for ECT.
- Multimeter for voltage/resistance checks.
- If possible, a way to compare actual coolant temp (IR thermometer or feel for thermostat opening).
Decision flow
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Confirm the ECT reading makes sense
- After overnight rest: ECT should match ambient within a few °C.
- If ECT is -40°C (or extremely low) that usually indicates an open circuit.
- If ECT is 130–150°C immediately on KOEO, that often indicates a short to ground (or sensor signal short).
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Check for companion codes / fan behaviour
Many vehicles command radiator fans as a fail-safe when ECT signal is implausible. That symptom supports a circuit fault.
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Connector + harness check at the ECT sensor
- Look for coolant intrusion, green corrosion, broken locks, or stretched wires near the plug.
- Wiggle-test the connector while watching live ECT for dropouts.
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Verify reference and ground (typical 2-wire sensor)
- Most ECT sensors are thermistors with a 5V reference and sensor ground from the ECU.
- Check for a stable reference voltage and a good ground at the connector (KOEO).
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Simple sensor test
- With sensor unplugged, ECT reading often defaults very low (open circuit). If it doesn’t change, suspect wiring/ECU input.
- Measure sensor resistance (cold vs warm): resistance should drop as temperature rises.
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Rule out a “real” cooling issue (only if the reading is plausible)
- If the sensor reads plausibly but the engine truly overheats, switch to the overheating flow.
- If the engine runs cool and sets thermostat-related faults, see P0128 flow.
Related: Overheating flow → · P0128 thermostat flow →
Related flows
Print / save checklist
Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.
- Freeze-frame captured / conditions noted
- KOEO ECT plausibility checked (vs ambient)
- Connector inspected (coolant intrusion/corrosion)
- Reference/ground checked at sensor connector
- Sensor resistance checked cold vs warm
- Confirm fix by reproducing the original condition
What to do next
Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.
Find another symptom flow
Jump to the symptom selector to locate the closest decision tree.
Workshop Guides
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