Powertrain, generic · Fuel and air metering

P0177 Fuel Composition Sensor Circuit Range/Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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. P0177 means the signal lies within the valid frequency range but is implausible – it jumps, changes without a refuelling event or does not match the mixture adaptation. Such sensors measure permittivity, conductivity and temperature and compare them with tables of known blends. GM's documents only list low, high and P2269, so no manufacturer threshold for P0177 was found; the description rests on CARB's rationality requirement and component information.

Likely causes

Sensor drifting or dirty inside
Water or contamination in the fuel
Loose contact in the connector
Control module or software

Symptoms

  • Check engine light on
  • Wrong ethanol value, stumbling or starting problems
  • Adaptation codes P0171/P0172

Repair estimate

DifficultyEasyDIY possible
Parts and labour€80–300
Time0.3–1 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel composition sensorsensor wiringpowertrain relay

Related codes

P0176P0178P0179P0169P2269

References: law.cornell.edu, patents.google.com, newtonnet.co.uk.