Technical Description of OBD-II Trouble Code
Range/Performance Issue with Fuel Composition Sensor Circuit
What does this mean?
This is a generic diagnostic trouble code (DTC) that applies to all OBD-II compatible vehicles. The specific repair steps may vary depending on the make and model of the vehicle.
This trouble code is specific to flex fuel vehicles. The fuel composition sensor, also known as a flex fuel sensor (FFS), measures the percentage of ethanol in the fuel supplied to the engine.
The powertrain control module (PCM) uses the FFS information to calculate the optimal air/fuel mixture and ignition timing for optimal performance and fuel efficiency under all operating conditions. The FFS produces a frequency (hz) signal that reflects the percentage of methanol in the fuel.
A higher percentage of methanol results in a higher frequency. If the PCM receives signals from the FFS that are outside of a specific range, it will activate the check engine light and set DTC P0177.
Other related trouble codes for the fuel composition sensor circuit include:
Symptoms & Severity of the Code
Symptoms of a P0177 code may include:
If the conditions for setting this code are present for two consecutive key cycles, the PCM will limit the amount of timing advance and potentially lower the rev limiter RPM.
Due to the potential negative effects on catalytic converters, fuel mileage, and power, it is recommended to have this code repaired as soon as possible.
Causes
The FFS has an operating range of 30hz to 145hz, with the normal range being 40hz to 125hz. The FFS circuit can be affected by electromagnetic interference caused by faulty secondary ignition components (such as spark plugs, spark plug wires, and ignition coils) and aftermarket audio equipment, resulting in a signal outside of the expected range and triggering DTC P0177.
Possible causes for this code include:
Diagnostic and Repair Procedures
I have encountered this code only once in the field, and it turned out to be a man-made problem. The customer had installed a 0 gauge audio amp power wire that ran directly over the harness containing the FFS circuit. The magnetic field created by the current running through the power wire interfered with the frequency output of the FFS, causing it to send a false signal of 145hz to the PCM.
While I have never come across a faulty FFS, it is a possible cause for this code, although less likely than electrical interference caused by aftermarket additions or faulty ignition components.
A PCM failure in the FFS circuit would be very rare.
