Investigate immediately. Check for real overheating. P0117 means the engine coolant temperature sensor is reporting a voltage below 0.14 V, which the ECM interprets as a short circuit in the signal wire corresponding to an implausibly high coolant temperature above 135 °C.
What P0117 means
The engine coolant temperature (ECT) sensor is a negative-temperature-coefficient (NTC) thermistor: its resistance falls as temperature rises, which in turn pulls the ECM's 5 V reference voltage down at the signal input terminal. Under normal conditions the sensor voltage ranges from 0.14 V to 4.91 V corresponding to coolant temperatures from approximately -60 °C to 135 °C. P0117 is a 1-trip fault — the ECM stores the code immediately if the ECT sensor voltage falls below 0.14 V for 0.5 seconds or more. At that voltage level the ECM calculates a coolant temperature above 135 °C, which is physically implausible under normal engine operation and therefore flagged as a short circuit. When P0117 is active the ECM enters fail-safe mode and substitutes a fixed 80 °C coolant temperature for all calculations, maintaining basic engine operation. The scan tool will display a coolant temperature above 135 °C while the fault is present.
Symptoms
- Check engine light on immediately — P0117 is a 1-trip code, MIL illuminates within 0.5 seconds of fault onset
- Scan tool displays coolant temperature above 135 °C or a fixed value that does not change with actual engine temperature
- Possible rich running and poor fuel economy in fail-safe mode if actual temperature differs significantly from the 80 °C substitute value
- Fan may run continuously — the ECM may command cooling fan on as a safety measure when it sees high ECT
- Hard starting in cold conditions — cold-start fuel enrichment depends on real ECT data; the 80 °C substitute skips cold-start enrichment
Common causes
- Short circuit in the ECT sensor signal wire to ground — the most direct electrical cause of a below-0.14 V reading
- Failed ECT sensor with internally shorted thermistor element — sensor resistance has collapsed below the normal range
- Actual engine overheating — a genuine coolant temperature above 135 °C will produce a voltage below the threshold and set this code alongside possible coolant system faults
- Corroded or damaged ECT sensor connector with the signal and ground terminals bridged
- ECM connector pin damage causing an internal short at the THW (coolant sensor signal) input
Severity & driving advice
Severity: high — Fail-safe substitutes 80 ?C ? cold-start enrichment skipped and fan logic impaired. If engine is actually overheating, continued driving risks serious damage.
Can I drive? Investigate immediately. Check for real overheating.
Diagnostic approach
- Read actual coolant temperature and compare to scan tool display — With the engine at operating temperature, the scan tool should show 75–100 °C (167–212 °F). If it displays above 135 °C (275 °F), the P0117 short-circuit path is confirmed as the cause. If the reading is -40 °C (-40 °F), the fault is actually an open circuit — check for P0118 instead.
- Check for actual overheating first — Inspect the coolant level in the reservoir and radiator. Feel the radiator hoses carefully (caution: scalding risk). If the engine is genuinely overheating, cooling system faults (failed thermostat, water pump, coolant leak) are the root cause of the P0117 reading — repair the cooling system, not the sensor.
- Disconnect the ECT sensor connector and test the wire harness for short to ground — With the sensor connector unplugged, connect terminals 1 and 2 of the harness-side connector together. The scan tool should then display above 135 °C — this confirms the wire harness itself is intact and can carry the shorted signal. If the display stays at a normal reading after jumpering, the open is in the harness between the sensor and ECM.
- Disconnect the sensor and check live scan tool temperature reading — With only the ECT sensor connector unplugged (not jumpered), the ECM should see an open circuit and display -40 °C. If the temperature display still shows above 135 °C with the sensor disconnected, the short to ground is in the wire harness or at the ECM connector — not inside the sensor.
- Verify ECM connector integrity if harness checks clear — Disconnect the ECM connector and connect the scan tool. With the connector unplugged, the scan tool should now read -40 °C. If the display still reads above 135 °C with both the sensor and ECM connectors disconnected, the ECM itself has an internal fault on the THW input pin and will require replacement.
- Replace the ECT sensor if wiring and ECM checks are normal — After replacing the sensor, perform the confirmation driving pattern: clear codes, start the engine, wait 0.5 seconds, check for DTCs. Also confirm the scan tool now reads a realistic coolant temperature that tracks engine warm-up from cold start to operating temperature (75–100 °C).
Make & model notes
Toyota: On Toyota 4Runner, Tacoma, and FJ Cruiser with the 1GR-FE V6, the ECT sensor is mounted in the engine block and uses a two-wire connector (THW signal and ETHW ground). The connector is exposed to engine heat and road spray, and the signal pin corrodes progressively — inspect the connector before condemning the sensor.
Ford: Ford EcoBoost engines (2.0L, 2.3L, 3.5L) route the ECT sensor wiring near the intake manifold where it can be pinched during DIY air filter service. Check the harness routing after any recent intake work before testing the sensor itself.
GM: GM LS-series engines (5.3L, 6.0L, 6.2L) in trucks use a coolant temperature sensor that threads into the intake manifold coolant passage. The plastic-bodied sensors on higher-mileage engines occasionally crack internally, causing intermittent short-to-ground conditions that produce P0117 only when the engine vibrates.
FAQ
Will P0117 damage the engine?
Not directly from the code itself. The fail-safe mode substitutes 80 °C so the engine continues to operate. However, if actual engine overheating is the underlying cause of P0117 and you continue driving, the overheating itself will cause engine damage — warped cylinder head, blown head gasket, or worse. Always rule out real overheating first.
Why does the scan tool show 135 °C or higher when the engine is cold?
The ECT sensor voltage below 0.14 V corresponds to a calculated temperature above 135 °C in the ECM's look-up table. The actual coolant temperature is irrelevant — the voltage is out of range, so the ECM reports the highest mapped value. The scan tool display reflects the sensor circuit fault, not actual coolant temperature.
How is P0117 different from P0115?
P0115 is a general circuit fault that can be triggered by either a voltage too high or too low. P0117 is specifically triggered only when the voltage is below 0.14 V (the short-circuit, or apparent over-temperature, threshold). P0118 is the open-circuit variant where voltage is above 4.91 V.