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DTC P0178

DTC P0178

Diagnostic Instructions


• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• Diagnostic Procedure Instructions provides an overview of each diagnostic category.


DTC Descriptor

DTC P0178

Fuel Composition Sensor Circuit Low Frequency

Circuit/System Description

Alcohol content and fuel temperature information is supplied to the ECM from the fuel composition sensor. The fuel composition sensor has a battery positive circuit, a signal circuit, and a ground circuit. The fuel composition sensor 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 ethanol content and the pulse width indicates fuel temperature. The ECM has an internal pull-up to 5 V on the signal circuit and the fuel composition sensor draws 5 V to ground in pulses. The normal range of operating frequency is between 50-150 Hz. The normal pulse width ranges between 1-5 milliseconds. The microprocessor within the sensor has certain self-diagnostics capabilities. An output frequency of 180 Hz indicates that the fuel is contaminated or that an internal sensor error has been detected.

Conditions for Running the DTC


• The ignition voltage is greater than 11 V.

• The DTC runs continuously when the above condition is met.


Conditions for Setting the DTC

The ECM detects that the fuel composition sensor output frequency is less than 45 Hz for greater than 6 s.

Action Taken When the DTC Sets

DTC P0178 is a Type B DTC.

Conditions for Clearing the MIL/DTC

DTC P0178 is a Type B DTC.

Diagnostic Aids


• The hertz and millisecond scan tool parameters react immediately to any change in the fuel composition sensor circuits, but may not react long enough to set a 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 fuel composition sensor circuits for an intermittent open.


Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference


• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Special Tools


• J 38522 Variable Signal Generator

• J 44175-A Fuel Composition Tester


For equivalent regional tools, refer to Special Tools .

Circuit/System Verification


1. Engine running, verify the scan tool Fuel Composition Sensor parameter is between 50-150 Hz.

2. Operate the vehicle within the conditions for running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.


Circuit/System Testing


1. Ignition OFF, disconnect the harness connector at the fuel composition sensor.

2. Ignition OFF and all vehicle systems OFF. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 5 Ω between the ground circuit terminal 2 and ground.

3. If greater than the specified range, test the ground circuit for an open/high resistance.

4. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal 1 and ground.

5. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the fuse is open, test all the components connected to the circuit, and repair or replace as necessary.

6. Ignition ON, test for 4.8-5.2 V between the signal circuit terminal 3 and ground.

7. If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K20 ECM.

8. If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the K20 ECM.

9. Ignition OFF, connect the red lead of the J 38522 Variable Signal Generator to the signal circuit terminal 3 at the fuel composition sensor harness connector. Connect the battery voltage supply to B+, and the black lead to ground.

10. Set the J 38522 Variable Signal Generator signal to 5 V, the frequency to 60 Hz, and the duty cycle to Normal.

11. Engine idling, verify the scan tool Fuel Composition Sensor parameter is at 60 Hz.

12. If not at the specified value, replace the K20 ECM.

13. If all circuits test normal, test or replace the fuel composition sensor.


Component Testing


1. Test the fuel composition using the J 44175-A Fuel Composition Tester and the J 44175-A Instruction Manual. Record the frequency.

2. Compare the special tool test frequency and the scan tool Fuel Composition Sensor frequency parameter. The difference between the tool and the sensor should be less than 12 Hz.

3. If greater than the specified value, replace the fuel composition sensor.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

Control Module References for ECM replacement, programming and setup