Fuel composition sensor fault (internal failure or contaminated sensor element); Wiring damage, short to voltage or chafing in the sensor signal circuit; Corroded, loose or damaged connector terminals at the sensor or ECU; Contaminated or poor quality fuel, such as water or an ethanol blend outside the sensor's range; Open circuit or fault in the sensor ground or reference supply; Fault in the engine control unit input circuit, or outdated calibration where the manufacturer has issued a fix for this fault.

P0179

Fuel Composition Sensor Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
79: FaultThe specific fault in that area

What it means

P0179 means the engine control unit has seen a signal voltage or frequency from the fuel composition sensor circuit that is higher than expected. This sensor reports the ethanol content (and sometimes fuel temperature) so the ECU can adjust fuelling and ignition timing. The code sets when the input stays above the acceptable range for long enough to be treated as a fault.

Symptoms

Malfunction indicator lamp on
Poor starting, especially when cold
Rough idle or hesitation under load
Poor fuel economy
Engine running rich or lean depending on how the ECU compensates
Possible fuel trim or other fuel system codes stored alongside

Possible causes

Fuel composition sensor fault (internal failure or contaminated sensor element)likely
Wiring damage, short to voltage or chafing in the sensor signal circuitlikely
Corroded, loose or damaged connector terminals at the sensor or ECUpossible
Contaminated or poor quality fuel, such as water or an ethanol blend outside the sensor's rangepossible
Open circuit or fault in the sensor ground or reference supplypossible
Fault in the engine control unit input circuit, or outdated calibration where the manufacturer has issued a fix for this faultrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including fuel temperature, engine load and coolant temperature.
Check for related fuel trim, misfire or sensor circuit codes that might point to a wider problem.
Inspect the sensor, its wiring harness and connector for damage, fuel leaks, corrosion, bent pins or chafing.
Ask when the car was last refuelled and check for the wrong fuel, water contamination or an unusual ethanol blend.
Use live data to compare the reported ethanol content and fuel temperature against what is realistic for the fuel in the tank.
Check the sensor supply, ground and signal circuit with a multimeter or oscilloscope against the manufacturer's wiring data, looking for shorts to voltage or poor grounds.
Check the sensor's signal output against the manufacturer's specification; the expected values depend on the model and sensor type.
Check for any manufacturer service bulletins or calibration updates for the vehicle before replacing the ECU.

How to fix

Replace the fuel composition sensor if it is faulty or contaminated.
Repair or replace damaged wiring, and clean or renew corroded connector terminals.
Drain and replace contaminated fuel, then clear the code and confirm it does not return.
Restore the sensor ground or supply circuit if a fault is found.
Update the ECU calibration only if the manufacturer lists an outdated calibration as a known cause, and repair or replace the ECU if its input circuit is proven faulty.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but the ECU may be using default fuelling and timing, which can affect running and economy. Get it checked soon, and avoid driving at all if the engine runs badly or you suspect contaminated fuel.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.