P0178 Code: Fuel Composition Sensor Circuit Low
🟠 Moderate severity · drivable
Drivable, but the engine may run slightly off because the computer's fuel mixture reading is unreliable.
What P0178 means
The fuel composition sensor on a flex-fuel vehicle is sending a voltage that is lower than the computer expects. A low reading usually means the sensor is failing, its wiring is shorted, or the connector is damaged. The computer can't reliably tell how much ethanol is in the fuel.
Symptoms
- Check engine light is on
- Hard starting or rough idle
- Worse fuel economy
- Usually no noticeable driving symptoms
Common causes
- most common: Faulty fuel composition sensor
- common: Wiring shorted to ground or damaged connector
- less common: Corroded connector pins
- rare: Faulty engine computer (PCM)
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Replace the fuel composition sensor | Mechanic | $150–400 |
| Repair or replace damaged wiring and connectors | Yes | $10–60 |
Frequently asked questions
Can I drive with a P0178 code?
Yes. The car should still run, but the computer may not know the correct fuel mix, which can affect cold starts.
What does "circuit low" mean?
The sensor's signal voltage is below the range the computer expects, usually because the sensor failed or its wiring is shorted.
What is the most common cause of P0178?
A failed fuel composition sensor, with wiring shorts close behind.
Related codes
P0176
P0176 Code: Fuel Composition Sensor Circuit Malfunction
P0177P0177 Code: Fuel Composition Sensor Range/Performance
P0179P0179 Code: Fuel Composition Sensor Circuit High
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.