Fuel and air

P0179 — Fuel Composition Sensor Circuit High Input

Severity: medium Drive with care, to a shop

What P0179 means

The engine control module monitors the fuel composition sensor, which measures the ethanol share in the fuel and usually the fuel temperature. It uses this to adjust fuel delivery and ignition timing. P0179 sets when the signal voltage stays above the allowed range, pointing to a failed sensor, an open circuit or a short to power in the wiring.

Symptoms

  • Check Engine Light on
  • Worse fuel economy
  • Rough idle
  • Hesitation or stumbling under acceleration
  • Reduced power
  • Hard start or no-start, especially after fuel module service

Possible causes

  1. Failed fuel composition sensor
  2. Damaged, corroded or unplugged sensor connector
  3. Open circuit in the signal wire
  4. Signal wire shorted to positive voltage
  5. Contaminated or poor-quality fuel skewing the reading
  6. Rarely, a faulty ECM

How to diagnose

  1. Scan all codes and freeze frame data, clear the code and see whether it returns
  2. Inspect the sensor connector and harness for corrosion, chafing and a fully latched plug
  3. If the fuel module or tank was recently serviced, confirm the connector at the tank is plugged in
  4. Check the scan tool's ethanol percentage and fuel temperature readings against the actual fuel in the tank
  5. Test power, ground and reference voltage at the sensor connector using the wiring diagram
  6. Test the signal wire for opens and for shorts to voltage
  7. Check the fuel for contamination
  8. If wiring is good, test or replace the sensor, and suspect the ECM only as a last resort

Common fixes

  • Replace the fuel composition sensor
  • Repair damaged wiring and clean or replace the connector
  • Drain contaminated fuel, refill with good fuel and replace the fuel filter
  • Update or replace the ECM if everything else checks out

Questions and answers

Can I drive with a P0179 code?

Usually yes, for short trips. The fuel mixture may be inaccurate, which can hurt economy and cause hesitation, so get it diagnosed soon.

What most often causes P0179?

Most often it is the sensor itself or a problem in its connector or wiring, such as an open, corrosion or a short to power. Check the connector and harness first, then replace the sensor if needed.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.