Circuit Description
Alcohol content and fuel temperature information is supplied to the powertrain control module (PCM) from the fuel composition sensor (FCS). The FCS has the following circuits:
| • | An ignition 1 circuit |
| • | A signal circuit |
| • | A ground circuit |
The FCS uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temperature and changes output signals accordingly. The signal circuit carries both the ethanol percentage and the fuel temperature within the same signal. The signal is both variable frequency and variable pulse width. The frequency signal indicates the ethanol percentage and the pulse width indicates fuel temperature. The PCM provides an internal pull up to 5 volts on the signal circuit and the FCS pulls the 5 volts to ground in pulses. The normal range of operating frequency is between 50-150 hertz. The normal pulse width ranges between 1-5 milliseconds. The microprocessor inside the sensor is capable of a certain amount of self-diagnosis. An output frequency of 170 hertz indicates that the fuel is contaminated or an internal sensor fault has been detected. If the PCM detects a signal more than 155 hertz, DTC P0179 will set.
DTC Descriptors
This diagnostic procedure supports the following DTC:
DTC P0179 Fuel Composition Sensor Circuit High Frequency
Conditions for Running the DTC
| • | The engine run time is more than 30 seconds. |
| • | The system voltage is greater than 10.9 volts. |
Conditions for Setting the DTC
The FCS frequency is more than 155 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. |
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views or Powertrain Control Module Connector End Views | ||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
2 |
Is the frequency greater than the specified value? | 155 Hz | Go to Step 4 | Go to Step 3 |
3 |
Did the frequency rise above the specified value? | 155 Hz | Go to Step 5 | Go to Testing for Intermittent Conditions and Poor Connections |
4 |
Is the different between the two frequencies less than the specified value? | 12 Hz | Go to Step 5 | Go to Step 6 |
5 | Clean the fuel system. Refer to Fuel System Cleaning . Did you complete the actions? | -- | Go to Step 7 | -- |
6 | Replace the FCS. Refer to Fuel Composition Sensor Replacement . Did you complete the replacement? | -- | Go to Step 7 | -- |
7 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 8 |
8 | Observe the Capture Info with a scan tool. Have any other DTCs not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK |
