Powertrain, generic · Fuel and air metering

P0178 Fuel Composition Sensor Circuit Low

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. 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.

Likely causes

Signal wire open or shorted to ground
Sensor without supply (relay, fuse, ground)
Sensor electrically failed (corrosion)
Engine control module

Symptoms

  • Check engine light on
  • Ethanol value 0 % or frozen
  • Starting problems on E85
  • GM: remote start disabled

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

P0176P0177P0179P2269P0169

References: law.cornell.edu, gsi.ext.gm.com, static.nhtsa.gov.