P0179

CIRCUIT DESCRIPTION
Alcohol content information is supplied to the PCM from the fuel composition sensor. The fuel composition sensor measures fuel temperature through an internal thermistor. The fuel conductivity and capacitance are electronically measured inside a small fuel cell. Internal circuitry combines the electrical measurement and fuel temperature information using a microprocessor to produce a Hertz frequency output.

Circuitry inside the sensor provides a default output of 170 Hz if the fuel conductivity is too high or an internal fault is detected. A test of the fuel must be made to determine the cause of the 170 Hz output.

CONDITIONS FOR RUNNING THE DTC
^ Engine run time is more than 30 seconds.
^ Ignition voltage is more than 10.9 volts.

CONDITIONS FOR SETTING THE DTC
The Fuel Composition Sensor (FCS) frequency is more than 155 Hz for 12.5 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The Powertrain Control Module (PCM) illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM turns the MIL OFF after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A last test failed (current DTC) clears when the diagnostic runs and does not fail.
^ A history DTC clears after 40 consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures.
^ Use a scan tool in order to clear the MIL/DTC.

DIAGNOSTIC AIDS
The Hz and ms parameters on the scan tool react immediately to any change of state in the ground or signal circuits. Hz and ms may show a change, but the fault may not occur long enough to set the code. Watch Hz and ms to identify intermittent problems in these two circuits. The Freeze Frame and Failure Records may be used to reproduce conditions at the time of the failure. A reading of 16 Hz in the Freeze Frame and Failure Records indicates a hard fault occurred.

The ignition positive voltage circuit needs a hard fault to set a code. An intermittent fault here may not show up by looking at Hz or ms. If the DTC will not reset and Hz and ms are steady, thoroughly inspect all FCS circuits while paying close attention to the ignition positive voltage circuit wire and or terminals for an intermittent open.

IMPORTANT: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

The following may cause an intermittent:
^ Mis-routed harness
^ Rubbed through wire insulation
^ Broken wire inside the insulation
^ Poor terminal contact

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
2. This step checks for a frequency that is out of range or intermittent at the time of the test.
3. This step determines if the intermittent high frequency is due to water droplets passing through the sensor.
6. This step compares the reading from the fuel composition sensor with an external test tool to determine the accuracy of the vehicle sensor and the compatibility of the fuel in the tank.