P0178
Fuel Composition Sensor Circuit Low
Fuel & air metering
Generic OBD-II
High
What it means
P0178 means the signal from the fuel composition (flex-fuel) sensor is too low, typically from a short or a grounded signal wire. That sensor tells the computer how much ethanol is in the fuel so it can adjust fueling. When the signal is lost the computer uses a default blend, so the car usually still runs but may idle rough or lose economy, particularly after refueling.
High
Can I keep driving? Usually still drivable — the computer falls back to a default fuel blend — but a flex-fuel vehicle may run rough or lose economy, especially after fueling up, so fix it soon.
Common symptoms
What you'll notice
- Check-engine light on
- Rough running, most noticeable after refueling or a change in fuel type
- Reduced fuel economy
- Occasional hesitation or hard cold start
Common causes
Most likely first
- Faulty flex-fuel (ethanol) composition sensorLikely
- Short-to-ground or low-side fault in the sensor signal wiringCommon
- Corroded or loose sensor connectorCommon
- Damaged harness in the fuel composition sensor circuitLess often
- Rarely, an ECM/PCM faultLess often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and check the reported ethanol percentage in live data.
- Inspect the fuel composition sensor connector and wiring for a short to ground or damage.
- Check the signal, power, and ground wires against spec.
- Replace the fuel composition sensor if the circuit checks out but the signal stays low.
- Clear the code and re-test, ideally after a fresh tank of fuel.
FAQ
Quick answers
Is it safe to drive with P0178?+
Usually yes. The computer uses a default fuel blend when it loses the composition signal, so the car runs — a flex-fuel vehicle may just run rough or lose economy after fueling. Repair it before it affects driveability further.
Can I just clear P0178?+
It returns until the sensor or its wiring is repaired. Because 'circuit low' usually means a short or grounded signal wire, inspect the harness before replacing the sensor.
Related codes
Often seen with P0178
Read & clear P0178 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