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 an ignition 1 circuit, a signal circuit, and 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 Hz. The normal pulse width ranges between 1-5 milliseconds. If the PCM detects a signal less than 45 Hz, DTC P0178 will set.
DTC Descriptors
This diagnostic procedure supports the following DTC:
DTC P0178 Fuel Composition Sensor Circuit Low Frequency
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.
Diagnostic Aids
| • | The hertz and millisecond parameters react immediately to any change in the FCS circuits, but may not react long enough to set this DTC. |
| • | A reading of 16 Hz in the Freeze Frame/Failure Records indicates a circuit failure. |
| • | An intermittent condition may not be detected because the sensor does not immediately power down and the connection may be restored before voltage drops below the operating threshold. |
| • | If this DTC will not reset and the parameters are steady, thoroughly inspect all FCS circuits for an intermittent open. |
| • | If a intermittent condition is suspected, refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
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 below the specified value? | 45 Hz | Go to Step 4 | Go to Step 3 | ||||||
3 |
Did the DTC failed this ignition? | -- | Go to Step 4 | Go to Diagnostic Aids | ||||||
4 |
Does the test lamp illuminate? | -- | Go to Step 5 | Go to Step 9 | ||||||
5 | Connect a test lamp between the ignition 1 voltage circuit and the ground circuit of the FCS harness connector. Does the test lamp illuminate? | -- | Go to Step 6 | Go to Step 10 | ||||||
6 | Measure the voltage from the fuel temperature/composition signal circuit to a good ground with a DMM . Is the voltage within the specified range? | 4.3-5.2 V | Go to Step 7 | Go to Step 8 | ||||||
7 |
Is the FCS Frequency parameter at the specified value? | 60 Hz | Go to Step 14 | Go to Step 11 | ||||||
8 | Test the fuel temperature/composition signal circuit for the following:
Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 16 | Go to Step 12 | ||||||
9 | Test the ignition 1 voltage circuit for an open or short to ground. Refer to Circuit Testing in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 16 | -- | ||||||
10 | Test the ground circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 16 | -- | ||||||
11 | Test the FCS signal circuit for an open or for high resistance. Refer to Circuit Testing in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 16 | Go to Step 12 | ||||||
12 | Test for an intermittent and for a poor connection at the powertrain control module (PCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring System. Did you find and correct the condition? | -- | Go to Step 16 | Go to Step 13 | ||||||
13 | Replace the PCM. Refer to Control Module References in Computer/Integrating Systems. Did you complete the replacement? | -- | Go to Step 16 | -- | ||||||
14 | Test for an intermittent and for a poor connection at the FCS. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 16 | Go to Step 15 | ||||||
15 | Replace the FCS. Refer to Fuel Composition Sensor Replacement . Did you complete the replacement? | -- | Go to Step 16 | -- | ||||||
16 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 17 | ||||||
17 | 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 | ||||||
