P0178: Fuel composition sensor signal is lower than the computer expects
Fuel composition sensor circuit low input
Severity: Monitor
The sources list a lit check engine light and, at most, poor economy or hard starting, with no safety warning, so it is usually a check-soon item.
P0178 in short
P0178 means the computer sees a low signal from the fuel composition sensor circuit. TroubleCodes.net lists a wiring short to ground, the sensor itself, or the engine computer as the likely causes. The sources do not describe symptoms, so have it checked rather than guessing.
A fuel composition sensor tells the computer what the fuel is made of, for example how much ethanol it contains. P0178 means the voltage from that sensor circuit is lower than the computer expects.
TroubleCodes.net lists a wiring short to ground, the sensor and the computer as probable causes. It has no detailed guide or symptom list for this code, so none is claimed here.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Wiring short to ground
Common general cause- Why it ranks here
- TroubleCodes.net lists a wiring short to ground first among probable causes.
- What would change this
- Rises if the signal wire tests shorted to ground. Falls if the circuit tests normal.
Cause 2: Failed fuel composition sensor
Common general cause- Why it ranks here
- TroubleCodes.net lists the sensor as a probable cause.
- What would change this
- Rises if the wiring tests good and the reading is stuck low. Falls if the wiring is damaged.
Cause 3: Engine computer fault
Common general cause- Why it ranks here
- TroubleCodes.net lists the engine computer as a probable cause; it ranks last because computer failures are less common.
- What would change this
- Rises only after the sensor and wiring both test good. Falls if either is faulty.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Read all stored codes and freeze-frame data with a scan tool and note any other fuel or sensor codes.
IfThis is the only code and the car drives normally
It meansMove on to checking the circuit.
Step 2: Have the fuel composition sensor wiring and connector tested for a short to ground with a multimeter, then check the sensor reading on a scan tool.
Warning:The sensor is usually part of the fuel system, so working on it risks fuel leaks and fire; leave it to a shop.
IfThe wiring tests good and the reading stays low
It meansThe sensor is the likely fault.
IfThe signal wire is shorted to ground
It meansRepair the wiring or connector.
Symptoms
- Check engine light on
How to fix
Step 1: Tell the shop whether you recently filled up with an unusual fuel such as a high-ethanol blend.
On your vehicle
We don't have reviewed vehicle-specific evidence for P0178 yet. The causes and tests above apply across vehicles.
Related codes
No curated relationships for P0178 yet.
Sources and review
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error