P0178 — Fuel Composition Sensor Circuit Low Input
Drive with caution — fix soon.
What P0178 means
This code means the voltage in the fuel composition sensor circuit has dropped below the normal range, signaling a low input. It's most often caused by a short to ground in the wiring, a poor connector, or a failed sensor. It's generally a sensor or wiring issue and is fixable once a technician traces the circuit and tests the sensor.
Common symptoms
- Check engine light is on
- Rough idle or slight engine shaking
- Reduced fuel economy
- Engine hesitation after refueling
- Slightly reduced power
Common causes
- Short to ground in the sensor wiring
- Failed fuel composition (flex-fuel) sensor
- Corroded or loose connector at the sensor
- Damaged or chafed wiring harness
- Faulty engine control module (rare)
Typical repair cost
$120 – $500
Varies by vehicle and root cause.
Frequently asked questions
Can I keep driving with a P0178 code?
Usually yes. The car typically still runs, but with the fuel reading off you may notice rough idle or lower mileage, so get it diagnosed within a week or two.
How much does it cost to fix a P0178?
A wiring or connector repair may run around $120, while replacing the fuel composition sensor with parts and labor can reach about $500.
Is a P0178 code serious?
It's moderate. It rarely strands you, but the engine relies on accurate fuel readings, so fixing it prevents drivability problems and wasted fuel down the road.
What does 'low input' mean for this sensor?
It means the circuit voltage is lower than normal, usually from a short to ground or a failed sensor. The computer flags this because it can't get a valid ethanol reading.