P0179
Fuel Composition Sensor Circuit High
Flex fuel / fuel composition
Generic OBD-II
High
What it means
P0179 means the signal from the fuel composition (flex-fuel) sensor is higher than the expected range. This sensor tells the PCM how much ethanol is in the fuel so it can adjust fueling and timing. With a faulty signal the PCM uses a default blend, so the car still runs but fueling may be slightly off — most often the fix is the sensor or its wiring.
High
Can I keep driving? Usually drivable — on a flex-fuel vehicle the ECM falls back to a default ethanol estimate, so expect at most rougher cold starts or slightly off fueling until it's fixed.
Common symptoms
What you'll notice
- Check-engine light on
- Rougher running or hesitation, especially after a fuel-blend change
- Slightly reduced fuel economy
- Occasional hard cold start on high-ethanol blends
Common causes
Most likely first
- Failed flex-fuel (fuel composition) sensorLikely
- Signal wire shorted high or an open in the circuitCommon
- Loose or corroded sensor connectorCommon
- Rarely, a PCM faultLess often
How to diagnose & fix
Work through it in order
- Inspect the fuel composition sensor connector and wiring for shorts to voltage, opens, or corrosion.
- Check the sensor's live reading — it should report a plausible ethanol percentage for the fuel in the tank.
- Verify the signal circuit has correct reference and ground back to the PCM.
- Replace the flex-fuel sensor if the reading stays pinned high with good wiring. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0179?+
Generally yes. The ECM uses a default fuel-blend value, so the car runs — though fueling may be slightly off until the sensor is fixed, especially on E85.
Can I just clear P0179?+
You can, but if the sensor or its wiring is faulty the code returns; repair the circuit or replace the sensor first.
Related codes
Often seen with P0179
Read & clear P0179 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX