P0178Fuel Composition Sensor Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- The fuel composition sensor is fitted only to flex-fuel vehicles that can run on petrol or ethanol up to E85. GM uses a capacitive sensor in the fuel line whose square-wave output frequency gives the ethanol content – 50 Hz for E0 up to 150 Hz for E100 – while the pulse width carries the fuel temperature. Ford has calculated the ethanol content from the oxygen sensor feedback after refuelling since model year 2000 and has no such sensor. P0178 means the frequency is too low. GM sets it below 45 Hz with 50 of 63 samples failing at 100 ms intervals (about 6 s), monitored continuously with the powertrain relay above 11 V; two trips. 'Low' refers to the frequency, not to the ethanol content: an open wire, missing supply or short to ground gives 0 Hz. The control module then uses a learned or substitute ethanol value. GM's road-salt special coverage for 2013–2014 flex-fuel models concerns exactly this kind of electrical failure.
- What you may notice
- Check engine light on
- Ethanol value 0 % or frozen
- Starting problems on E85
- GM: remote start disabled
- Likely causes
- LikelySignal wire open or shorted to ground
- LikelySensor without supply (relay, fuse, ground)
- LikelySensor electrically failed (corrosion)
- RareEngine control module
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €80 – €300
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.