P0117 on the 7.3L Power Stroke is an engine coolant temperature (ECT) sensor circuit low input fault. The ECT sensor reports how hot the engine is to the PCM, which uses that value for glow plug operation, injector timing, idle strategy, and fueling. When the PCM reads an ECT voltage too low, corresponding to an implausibly high coolant temperature, it stores P0117.
Even though the 7.3L is a comparatively simple engine, it still leans on coolant temperature to manage cold-start glow plug time and injection timing. A false hot reading can cut glow plug operation short and make cold starts harder, so P0117 is worth correcting even when the truck seems to run fine. Because the sensor is a thermistor, the fault almost always lives in the wiring, the connector, or the sensor itself.
What P0117 means
The ECT sensor is a thermistor whose resistance drops as coolant heats. Low resistance yields low voltage, which the PCM reads as very hot coolant. P0117 sets when the voltage stays at the low end, indicating a signal wire shorted to ground, a sensor that has failed shorted, or a connector bridging the signal to ground.
Common causes
- Signal wire shorted to ground
- Failed ECT sensor reading shorted
- Corroded connector bridging signal and ground
- Chafed or pinched harness near the sensor
- Coolant intrusion into the sensor connector
Symptoms
- Check engine light with P0117 stored
- Hard cold starting from reduced glow plug time
- Altered injection timing and idle behavior
- Temperature gauge reading that seems wrong
- Possible cooling fan behaving abnormally
How to diagnose it
Watch live ECT data with a scan tool. A reading pinned very hot when the engine is actually cold confirms the low circuit fault. Inspect the sensor connector for corrosion, coolant, and damaged pins. Unplug the sensor and watch the reading; if it swings to the opposite extreme, the wiring is good and the sensor is likely shorted. Check the signal wire for a short to ground and confirm continuity back to the PCM. Compare sensor resistance to a temperature chart to confirm.
How to fix it
Repair a signal wire shorted to ground and clean or replace a corroded connector, which fixes many P0117 cases. If the wiring checks good but the sensor reads shorted, replace the ECT sensor. Confirm the connector is sealed to prevent coolant or water intrusion. After the repair, clear the code, verify the ECT reading rises normally from a cold start, and road test. For the correct ECT sensor for your 7.3L Power Stroke, contact sales@coredieselparts.com.
Frequently asked questions
Will P0117 make my 7.3L hard to start?
It can. If the PCM thinks the engine is already hot, it shortens glow plug time, which makes cold starts harder and smokier until the sensor is fixed.
Is P0117 the sensor or the wiring?
Both are possible. Unplug the sensor and watch the reading. A swing to the opposite extreme means the wiring is good and the sensor is shorted; no change means a wire shorted to ground.
Can low coolant cause P0117?
Low coolant does not cause this specific code, which is an electrical low reading. However, always confirm coolant level and condition while you are diagnosing temperature-related faults.
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