P0179 code meaning
Fuel Composition Sensor Circuit High
Powertrain system Generic code P01xx codes Check engine light
What P0179 means
The fuel composition sensor is fitted only to flex-fuel vehicles that can run on gasoline 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. P0179 means the frequency is above the physically possible range for E100 plus tolerance. GM sets it between 155 and 185 Hz (50 of 63 samples, two trips); above 185 Hz it sets P2269 instead, because such a high value indicates water or conductive fuel rather than a sensor fault. Typical causes of P0179 are failed sensor electronics or interference on the signal wire. With a falsely high ethanol value the mixture tends to run rich.
P0179 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1 hours
- Clears after repairAfter a drive cycle
Symptoms of P0179
- Check engine light on
- Ethanol value implausibly high (100 %)
- Rich mixture, stumbling, higher consumption
Common causes of P0179
- Most likelySensor electronics faulty
- PossibleInterference or short on the signal wire
- PossibleWater or conductive fuel near the P2269 limit
- Less likelyEngine control module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Fuel composition sensorSensor wiringPowertrain relay
What each part of P0179 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 1 | Area: fuel and air metering. |
| 79 | The exact fault in that area. |
What to do next
- Note P0179 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0179
What does P0179 mean?
P0179 means "Fuel Composition Sensor Circuit High". The fuel composition sensor is fitted only to flex-fuel vehicles that can run on gasoline or ethanol up to E85. It is a generic code, so it means the same on every make.
Is it safe to drive with P0179?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0179 fail an emissions test?
Yes, it can. P0179 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0179 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0179 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.