P0178 Fuel composition sensor: signal or voltage too low

By: Diagno editorial team · Updated: 2026-10-15

P0178 means: Fuel composition sensor: signal or voltage too low. The computer found a fault in a sensor, valve or control circuit of this system. Drive gently and have the car checked soon. The repair usually costs about €40–300 in Estonia. The most common cause: connector loose, damp or corroded.

What it means

The computer found a fault in a sensor, valve or control circuit of this system. The signal voltage is too low. Usually a wire is broken, a connector is loose or a wire is shorted to ground.

What you may notice

• Engine light on
• A function doesn't work properly

What could be wrong

1. 1Connector loose, damp or corroded
2. 2Wire broken or shorted to ground
3. 3Faulty sensor or valve
4. 4Engine computer fault (rare)

Repair difficulty: Moderate

What you can check yourself

First look at the connector and wiring: anything loose, melted or green with corrosion? Check the connector concerned and see whether the fault returns after clearing it. You can check the simple things yourself, but the repair is best done in a workshop.

How we fix it

1. We read the fault data on the diagnostic tool and check whether the fault is constant
2. We measure the circuit's supply, ground and signal
3. We repair the wire or replace the connector
4. If the wiring is fine, we replace the sensor or valve

Standard SAE description: Fuel Composition Sensor Circuit Low Input