P0178
DTC P0178
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC P0178
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Fuel Composition Sensor Circuit Low Frequency
Circuit/System Description
Alcohol content 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 changes the output signal accordingly. The signal circuit carries the ethanol percentage via the frequency signal. The ECM provides an internal pull up to 5 V on the signal circuit, and the fuel composition sensor pulls the 5 V to ground in pulses. The normal range of operating frequency is between 50-150 Hz. The microprocessor inside the sensor is capable of a certain amount of self-diagnosis. An output frequency between 180 Hz and 190 Hz indicates that the fuel is contaminated.
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 Fuel Composition Sensor scan tool parameter reacts immediately to any change in the fuel composition sensor circuits, but may not react long enough to set a DTC.
* 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 (LE9) [1][2]Electrical Diagrams
Connector End View Reference
Component Connector End Views Sunshade - Left (DH6)
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References 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 (Diagnostic Tools) Tools and Equipment.
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 B198 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 ohms between the ground circuit terminal 2 and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal 1 and ground.
• 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.
4. Ignition ON, test for 4.8-5.2 V between the signal circuit terminal 3 and ground.
• 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.
• If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the K20 ECM.
5. Ignition OFF, connect the red lead of the J 38522 - Variable Signal Generatorto the signal circuit terminal 3 at the B198 fuel composition sensor harness connector. Connect the battery voltage supply to B+, and the black lead to ground.
6. Set the J 38522 - Variable Signal GeneratorSignal to 5 V, the Frequency to 60 Hz, and the Duty Cycle to Normal.
7. Ignition ON, verify the scan tool Fuel Composition Sensor parameter is at 60 Hz.
• If not at the specified value, replace the K20 ECM.
8. If all circuits test normal, test or replace the B198 fuel composition sensor.
Component Testing
1. Test the fuel composition using the J 44175-A - Fuel Composition Testerand 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.
• If greater than the specified value, replace the B198 fuel composition sensor.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the repair.
Control Module References Control Module References for ECM replacement, programming, and setup