Fuel Composition Sensor Circuit High
What does P0179 mean?
The P0179 code means the vehicle's onboard diagnostic system has detected: Fuel Composition Sensor Circuit High. This is a moderate severity code.
Common Symptoms
- Check Engine Light on
- Engine running rich (PCM thinks fuel has more ethanol than actual)
- Black smoke from exhaust
- Poor fuel economy
- Fouled spark plugs
Probable Causes (Ranked by Likelihood)
- Failed fuel composition sensor with high output Very High
- Short to voltage in the signal wire High
- Water contamination in fuel elevating sensor readings Moderate
- Corroded connector causing signal interference Moderate
- PCM input circuit fault Low
Step-by-Step Diagnostic Procedure
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Read fuel composition sensor voltage on a scan tool. A high reading when running regular gasoline (E10) confirms the sensor is over-reporting ethanol content.
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Disconnect the sensor and check if the high voltage drops. If it does, the sensor is at fault. If it remains high, check for a wiring short to power.
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Check for water in the fuel by draining a sample from the bottom of the fuel tank.
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Inspect the wiring for shorts to other power circuits or damaged insulation.
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If the sensor and wiring test good, check for fuel contamination or an incorrect fuel type.
Frequently Asked Questions
Can I drive with code P0179?
You can drive short distances. A high reading causes the PCM to inject too much fuel, running the engine rich. This wastes fuel, can foul spark plugs, and produces excess emissions.
What does high circuit mean for P0179?
High circuit means the fuel composition sensor is sending a voltage above the expected range to the PCM. This causes the PCM to think there is more ethanol in the fuel than there actually is, leading to over-fueling.
Can accidentally filling with E85 cause P0179?
On a non-flex-fuel vehicle, yes. The high ethanol content will cause sensor and fuel system issues. On a flex-fuel vehicle, the sensor should correctly read E85, so a high circuit code would indicate a sensor or wiring fault.