Engine Oil Temperature Sensor Circuit Low (Subaru)
What does P1178 mean?
The P1178 code means the vehicle's onboard diagnostic system has detected: Engine Oil Temperature Sensor Circuit Low (Subaru). This is a moderate severity code.
Common Symptoms
- Check Engine Light on
- Oil temperature reads abnormally high on scan tool
- Engine protection mode may activate
- Reduced power warning possible
- Dashboard oil warning may illuminate
Probable Causes (Ranked by Likelihood)
- Short to ground in oil temperature sensor circuit Very High
- Internal fault in oil temperature sensor High
- Damaged wiring between sensor and ECM Moderate
- Water intrusion in sensor connector Moderate
- ECM issue Low
Step-by-Step Diagnostic Procedure
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Read oil temperature sensor data with a scan tool (low circuit = PCM thinks oil is very hot)
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Disconnect the sensor and check if the reading changes to maximum
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Test sensor resistance with a multimeter at known temperature
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Check wiring for short to ground
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Replace the sensor if resistance is out of specification
Frequently Asked Questions
What does low circuit mean for the oil temperature sensor?
A low voltage circuit means the sensor resistance is too low, which the ECM interprets as very high oil temperature. This may trigger engine protection measures even if the oil is actually at a normal temperature.
Can P1178 cause the engine to go into limp mode?
On some Subaru models, yes. If the ECM believes the oil is dangerously hot based on the low circuit reading, it may limit engine power to protect the engine from perceived overheating.
Is the oil temperature sensor the same as the oil pressure sensor?
No. The oil temperature sensor measures how hot the oil is, while the oil pressure sensor measures oil pressure in the system. They are separate sensors at different locations on the engine.