P0178

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

CONDITIONS FOR RUNNING THE DTC
^ The engine run time is more than 30 seconds.
^ The ignition voltage is more than 10.9 volts.

CONDITIONS FOR SETTING THE DTC
The FCS frequency is below 45 Hz for 12.5 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.

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 change may not occur long enough to set the DTC. Watch Hz and ms to identify intermittent problems in the signal circuit. An intermittent fault in the signal circuit will immediately change the Hz and ms parameters. The Freeze Frame/Failure Records may be used to reproduce conditions at the time of the failure. A reading of 16 Hz in the Freeze Frame/Failure Records indicates a circuit fault occurred.

The ignition positive voltage circuit needs the fault to occur a longer period of time to affect the sensor output. An intermittent fault here may not show up by looking at Hz or ms because the sensor does not immediately power down and the connection may be restored before voltage drops below the operating threshold, such as a loose terminal. 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.

The fuel composition tester can be used to inspect the accuracy of the sensor if a fault is not isolated using this table. If the Fuel Test is performed before the electrical circuits have been tested using this table, the results will be in error.

IMPORTANT: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Verify 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

TEST DESCRIPTION

Steps 1-13:

Steps 14-15:

The numbers below refer to the step numbers on the diagnostic table.
2. This step verifies that a fault is present.
6. This step is observing for a reference voltage from the PCM. An excessively high or low voltage is an indication of a problem with the circuit, or a malfunctioning PCM.