Rick's Free Auto Repair Advice

P0179 GM Vehicles

Fix code P0179 on GM Flex Fuel Vehicles

If you own a GM Flex Fuel Vehicle and get a P0179- Fuel Composition Sensor (FCS) out of range, along with a whole host of misfire and oxygen sensor related codes, focus all your attention on the problem prone FCS first. The FCS sensor flex fuel sensoris designed to detect how much ethanol is in the gas so it can recalibrate spark timing and air/fuel ratios.

Start your diagnostics at the FCS on the left frame rail. Disconnect the electrical connector from the sensor. Using a DVOM, check for battery voltage on the pink wire. If you don’t get power, check the ECM 1 fuse. If the fuse is good, you’ve got a wiring problem and will have to backtrack it to the ECM fuse.

Next, check for good sensor ground on the purple wire provided by the powertrain control module (PCM). If that checks out, look for a reference voltage (5 volts) from the (PCM) on the white wire.

If all those readings check out, then you’ve got a tricky test that requires either a scope or a high end DVOM. The sensor puts out a variable 0-5 volt wave to tell the PCM the ethanol content. A 50Hz reading indicates 0% ethanol and a 100%Hz wave indicates 100% ethanol. If there’s a problem with the sensor itself, it will default to 170Hz. However, a 170Hz reading can also mean contaminated fuel or fuel with methanol in it.

Make sure you do all the electrical tests first before condemning the sensor—it’s VERY pricey–like $600. !!!!!


© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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