Use this procedure to troubleshoot the following diagnostic codes:
- 1088-5 Oxygen Sensor Power Supply current below normal
- 1088-6 Oxygen Sensor Power Supply current above normal
The oxygen buffer uses an oxygen sensor in order to measure the concentration of oxygen in the engine exhaust.
The oxygen buffer receives power from terminal P2-13 of the Electronic Control Module (ECM). The return circuit is through terminal P2-23. The buffer provides a power supply of 1.5 amperes and 10.5 volts between the positive VH and negative VH terminals in order to power a resistive type of heater in the oxygen sensor. After this heater raises the temperature of the sensing element to 700 °C (1292 °F), the buffer regulates the current from the positive IP terminal to the negative IP terminal. This keeps the voltage between the positive VS terminal and negative VS terminal relatively constant.
The amount of current is a measure of the concentration of oxygen in the exhaust gases. The current will vary from 0 mA at 0 percent of oxygen to approximately 8.7 mA at 21 percent of oxygen. The oxygen buffer measures this current. The buffer converts the current to a Pulse Width Modulated signal (PWM) with a nominal frequency of 500 Hz. This signal is supplied to terminal P2-67.
The duty cycle of the PWM signal has a range of 30 to 65 percent. The 30 percent corresponds to 0 percent of oxygen and the 65 percent corresponds to 21 percent of oxygen. These values vary slightly between different buffers and sensors. Calibration is required for every new buffer. Calibration of the buffer is also required after every 1500 service hours. Some applications may require more frequent calibration or less frequent calibration.
The oxygen buffer is equipped with a diagnostic circuit that detects the current which is provided to the heating element of the oxygen sensor. If this current is too low, the buffer assumes that the oxygen sensor has failed or the sensor is disconnected from the oxygen buffer. This will cause the PWM signal from the oxygen buffer to change to a duty cycle of approximately 15 percent. The ECM interprets this condition as a diagnostic condition. The ECM will generate a diagnostic code that can be viewed with the Caterpillar Electronic Technician (ET).
The sensor provides a wet reading that is slightly lower than a dry reading. The wet reading is multiplied by a conversion factor of approximately 1.25 in order to obtain the reading that is displayed on Cat ET. This allows oxygen readings from Cat ET to be compared to dry readings from an emissions analyzer. The ECM uses the signal of the percent of oxygen to maintain the desired emissions levels.
If an incorrect signal is detected by the ECM, a diagnostic code is activated and compensation for fuel quality is disabled. The ECM commands the fuel actuator to set the fuel valve to 70 percent open. Problems with the circuit for the oxygen signal must be repaired as soon as possible because of the effects on engine performance and emissions.
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| Illustration 1 | g01256771 |
Schematic of the circuit for the oxygen buffer and the oxygen sensor | |
Test Step 1. Inspect the Electrical Connectors and the Wiring
- Set the engine control to the OFF/RESET position.
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During operation, the oxygen sensor may reach temperatures that exceed 700 °C (1292 °F). Severe personal injury and property damage will result from contact with a hot oxygen sensor. Do not touch the sensor during engine operation, calibration, or testing. Allow the sensor to cool before moving the sensor. Wear gloves that resist heat. Do not place the sensor on or near any flammable material or any surface that can be damaged by high temperatures. |
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- Thoroughly inspect connectors (1), (3), (4), and (5) .
- Check the allen head screw for each of the ECM connectors for the proper torque.
- Perform a 45 N (10 lb) pull test on each of the wires that are associated with the circuit for the oxygen sensor.
Show/hide table
Illustration 4 g01392118
Terminal locations at the P2 ECM connector for the oxygen sensor
(P2-13) +Battery
(P2-23) Return
(P2-53) Shield
(P2-67) Signal from the oxygen sensor
- Check the harness and wiring for abrasion and for pinch points from the oxygen sensor to the ECM.
Note: For the following steps, refer to Troubleshooting, "Inspecting Electrical Connectors".
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| Illustration 2 | g01392084 |
Location of the ECM connectors (typical left front engine view) (1) J2/P2 ECM connectors | |
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| Illustration 3 | g01392092 |
Location of the components that are for the oxygen sensor (typical rear engine view) (2) Oxygen buffer (3) Harness connector for the oxygen buffer (4) Connector for the oxygen sensor at the oxygen buffer (5) Connector at the oxygen sensor | |
Expected Result:
All of the connectors, pins, and sockets are connected properly. The connectors and the wiring do not have corrosion, abrasion, or pinch points.
Results:
- OK - All of the connectors, pins, and sockets are connected properly. The connectors and the wiring do not have corrosion, abrasion, or pinch points. The components are in good condition with proper connections. Proceed to Test Step 2.
- Not OK - At least one of the connectors, pins, or sockets are not connected properly. At least one of the connectors and/or the wiring has corrosion, abrasion, and/or pinch points.
Repair: Perform the necessary repairs and/or replace parts, if necessary.
STOP
Test Step 2. Check for Diagnostic Codes for the Oxygen Buffer
- Establish communication between Cat ET and the ECM. Refer to Troubleshooting, "Electronic Service Tools", if necessary.
- Access the "Logged Diagnostic Codes" screen on Cat ET. If the following codes are logged, clear the codes:
- 1088-5 Oxygen Sensor Power Supply current below normal
- 1088-6 Oxygen Sensor Power Supply current above normal
- 1088-5 Oxygen Sensor Power Supply current below normal
- Start the engine and run the engine. Engine speed must be present in order to activate diagnostic codes. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".
- Select "Oxygen Sensor Override" under the "Description" column.
- Select "Change" in the lower left corner of the screen. A popup window with the title "Change Parameter Value" will appear.
- Select a "New Value" of "On" and then select "OK". Allow a minimum of ten seconds for the diagnostic codes to be activated.
The oxygen buffer is now on. The "Value" of the "Oxygen Sensor Override" should change to "On".
Note: After you exit the "Override Parameters" screen, the "Value" of the "Oxygen Sensor Override" is automatically changed to "OFF" when the engine control is not in the START position.
- Access the "Logged Diagnostic Codes" screen again. Look for the 1088-5 or 1088-6 diagnostic codes again.
Expected Result:
A 1088-5 or 1088-6 diagnostic code is logged.
Results:
- Logged 1088-5 diagnostic code - The supply for the buffer has an open circuit or a short circuit to the +Battery side. Proceed to Test Step 3.
- Logged 1088-6 diagnostic code - The supply for the buffer has a short circuit to ground. Proceed to Test Step 4.
- No codes - The supply for the buffer is OK.STOP
Test Step 3. Check the Supply Voltage for the Oxygen Buffer
- Set the engine control to the OFF/RESET position.
- Disconnect the harness connector from the oxygen buffer.
- Set the engine control to the Start/Run position.
- Start the engine and run the engine. Engine speed must be present in order to activate diagnostic codes. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".
- Select "Change" in the lower left corner of the screen.
- Select a "New Value" of "On" and then select "OK".
The oxygen buffer is now on. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.
Note: After you exit the "Override Parameters" screen, the status of the "Oxygen Sensor Override" is automatically changed to "OFF" when the engine control is not in the START position.
- Measure the voltage between terminals A and B.
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| Illustration 5 | g00894897 |
Harness connector for the oxygen buffer (A) +24 VDC for the buffer (B) Return | |
Expected Result:
The supply voltage for the oxygen buffer is greater than 18 VDC.
Results:
- OK - The supply voltage for the oxygen buffer is greater than 18 VDC. The buffer is receiving the correct supply voltage.
Repair: Connect the connectors. Determine whether the diagnostic code recurs. If the problem is not resolved, set the engine control to the OFF/RESET position. Replace the oxygen buffer. After you replace the buffer, calibrate the oxygen sensor. Follow the instructions in Troubleshooting, "Oxygen Sensor - Calibrate".
STOP
- Not OK - The supply voltage is less than 18 VDC. Proceed to Test Step 5.
Test Step 4. Disconnect the Oxygen Buffer and Check for a Short to Ground
- Set the engine control to the OFF/RESET position.
- Disconnect the harness connector from the oxygen buffer.
- Set the engine control to the COOLDOWN/STOP position.
- Access the "Logged Diagnostic Codes" screen on Cat ET. If a 1088-6 diagnostic code is logged, clear the code.
- Start the engine and run the engine. Engine speed must be present in order to activate diagnostic codes. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".
- Select "Change" in the lower left corner of the screen.
- Select a "New Value" of "On" and then select "OK". Wait for a minimum of 65 seconds.
The oxygen buffer is now on. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.
Note: After you exit the "Override Parameters" screen, the status of the oxygen sensor is automatically changed to "OFF" when the engine control is not in the START position.
- Access the "Logged Diagnostic Codes" screen again. Look for the 1088-6 diagnostic code.
Expected Result:
There is a logged 1088-6 diagnostic code.
Results:
- Yes - The supply for the buffer still has a short circuit. Proceed to Test Step 6.
- No - The supply for the buffer does not have a short circuit when the buffer is disconnected. The harness to the buffer is OK.
Repair: Set the engine control to the OFF/RESET position.Inspect the harness connector for corrosion and/or damage. Make repairs, as needed.If the condition is not resolved, replace the buffer. Be sure to calibrate the oxygen sensor after you replace the buffer. Refer to Troubleshooting, "Oxygen Sensor - Calibrate".
STOP
Test Step 5. Create a Short Circuit in the Harness
- Set the engine control to the OFF/RESET position.
- Disconnect the harness connector from the oxygen buffer.
- Install the ends of a jumper wire into terminals A and B.
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| Illustration 6 | g00894897 |
Harness connector on the harness for the oxygen buffer (A) +24 VDC for the buffer (B) Return | |
- Set the engine control to the COOLDOWN/STOP position.
- Access the "Logged Diagnostic Codes" screen on Cat ET. If a 1088-5 diagnostic code is logged, clear the code.
- Start the engine and run the engine. Engine speed must be present in order to activate diagnostic codes. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".
- Select "Change" in the lower left corner of the screen.
- Select a "New Value" of "On" and then select "OK". Wait for 65 seconds.
The oxygen buffer is now on. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.
Note: After you exit the "Override Parameters" screen, the status of the oxygen sensor is automatically changed to "OFF" when the engine control is not in the START position.
- Access the "Logged Diagnostic Codes" screen again. Look for the 1088-5 diagnostic code.
Expected Result:
The 1088-5 diagnostic code is not logged when the circuit for the supply to the buffer is shorted by the jumper wire.
Results:
- OK - The 1088-5 diagnostic code is not logged when the circuit for the supply to the buffer is shorted by the jumper wire. The harness and the ECM are OK.
Repair: Perform the following steps:
- Set the engine control to the OFF/RESET position.
- Inspect the harness connectors for corrosion and other damage. Make repairs, as needed.
If the condition is not resolved, replace the oxygen buffer. Be sure to calibrate the oxygen sensor after you replace the buffer. Refer to Troubleshooting, "Oxygen Sensor - Calibrate".
STOP
- Not OK - The 1088-5 diagnostic code is logged when the circuit for the supply to the buffer is shorted by the jumper wire. The ECM does not detect the short circuit. Proceed to Test Step 6.
Test Step 6. Check the Supply for the Oxygen Buffer at the ECM
- Set the engine control to the OFF/RESET position.
- Remove terminals P2-13 and P2-23.
This will create an open circuit between the buffer and the ECM.
- Set the engine control to the COOLDOWN/STOP position.
- Access the "Logged Diagnostic Codes" screen on Cat ET. If a 1088-5 or 1088-6 diagnostic code is logged, clear the code.
- Start the engine and run the engine. Engine speed must be present in order to activate diagnostic codes. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".
- Select "Change" in the lower left corner of the screen.
- Select a "New Value" of "On" and then select "OK". Wait for a minimum of 65 seconds.
The oxygen buffer is now on. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.
Note: After you exit the "Override Parameters" screen, the status of the oxygen sensor is automatically changed to "OFF" when the engine control is not in the START position.
- Install a jumper wire into terminals P2-13 and P2-23. Pull on the jumper wire in order to verify proper installation.
This will create a short circuit between the supply and the return of the buffer circuit.
- Access the "Logged Diagnostic Codes" screen again. Allow a minimum of ten seconds for any diagnostic codes to be activated. Look for the 1088-5 or 1088-6 diagnostic code.
Expected Result:
When an open circuit was created, a 1088-5 diagnostic code was activated.
When a short circuit was created, a 1088-6 diagnostic code was activated.
Results:
- Yes - When an open circuit was created at the P2 connector, a 1088-5 diagnostic code was activated. When a short circuit was created at the P2 connector, a 1088-6 diagnostic code was activated. The ECM is OK. There is a problem in the harness between the ECM and the oxygen buffer.
Repair: Perform the following procedure:Set the engine control to the OFF/RESET position.Remove the jumper wire from the P2 connector.Install the terminals into P2-12 and P2-23. Pull on the wires in order to verify proper installation of the terminals.Repair the harness, when possible. Replace parts, if necessary.
STOP
- No - The ECM did not detect an open circuit and/or a short circuit. There may be a problem with the ECM.
Repair: Perform the following procedure:Set the engine control to the OFF/RESET position.Remove the jumper wire from the P2 connector.Install the terminals into P2-12 and P2-23. Pull on the wires in order to verify proper installation of the terminals.Temporarily install a new ECM. Refer to Troubleshooting, "Replacing the ECM".If the problem is resolved with the new ECM, install the original ECM and verify that the problem returns. If the new ECM operates correctly and the original ECM does not operate correctly, replace the original ECM. Refer to Troubleshooting, "Replacing the ECM".
STOP