Open circuit, short to earth or chafed wiring in the fuel composition sensor signal wire; Corroded, loose or damaged sensor connector terminals; Faulty fuel composition sensor; Poor or missing power supply or earth to the sensor; Contaminated or unsuitable fuel, such as water in the fuel or a fuel type outside the sensor's range; Fault inside the engine control unit.

P0178

Fuel Composition Sensor Circuit Low.

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
78: FaultThe specific fault in that area

What it means

P0178 is the generic code for a low signal from the fuel composition sensor circuit, which measures the ethanol content (and sometimes the temperature) of the fuel. The engine control unit has seen the sensor's signal voltage or frequency fall below the expected range, so it cannot trust the fuel composition reading it uses to adjust fuelling and ignition timing.

Symptoms

Malfunction indicator lamp on
Poor starting or long cranking, especially when cold
Rough idle or hesitation under load
Poor fuel economy
Engine running rich or lean, with possible fuel trim faults stored alongside

Possible causes

Open circuit, short to earth or chafed wiring in the fuel composition sensor signal wirelikely
Corroded, loose or damaged sensor connector terminalslikely
Faulty fuel composition sensorlikely
Poor or missing power supply or earth to the sensorpossible
Contaminated or unsuitable fuel, such as water in the fuel or a fuel type outside the sensor's rangepossible
Fault inside the engine control unitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including engine temperature and fuel trims, then check for related fuel trim or sensor codes.
Check what fuel was last used and whether the car has been refuelled recently, as the reading may be affected by mixed or contaminated fuel.
Visually inspect the sensor, its wiring and connector for damage, heat, rubbing, fuel leaks or corrosion.
Check live data for the ethanol content and fuel temperature readings and see whether they are implausible, stuck or at the low limit.
With the connector disconnected, check the sensor's supply voltage and earth against the manufacturer's data for that model.
Test the signal wire for continuity, and for short to earth, back to the control unit.
If wiring and supply are good, check the sensor against specification or test with a known good unit, following the manufacturer's procedure.
Check for a relevant software update or technical bulletin for the vehicle, as the layout and sensor type vary by model and engine.

How to fix

Repair or replace damaged wiring and fix any short to earth or open circuit.
Clean or replace corroded or damaged connector terminals and secure the connector.
Restore the sensor's power supply or earth if either is faulty.
Replace the fuel composition sensor if it fails testing.
Drain and refill with fresh, suitable fuel if contamination is confirmed, and replace affected filters if required.
Replace or repair the control unit only after all other causes are ruled out and the fault is confirmed.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but the control unit may be using fallback fuelling, which can cause poor running and higher consumption. Get it checked soon, and avoid it if it runs very rough or stalls.

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