Use this procedure to troubleshoot the following diagnostic codes:
- 1087-3 Oxygen Sensor Buffer Module voltage above normal
- 1087-8 Oxygen Sensor Buffer Module abnormal frequency, pulse width, or period
The oxygen sensor is used by the oxygen buffer in order to measure the concentration of oxygen in the engine exhaust.
The oxygen buffer receives power from terminal P2-13 on the Electronic Control Module (ECM). The return circuit is through terminal P2-23. The buffer provides a power supply of 1.5 A and 10.5 V between the positive VH and negative VH terminals. The power is for 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. 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 60 percent. The 30 percent corresponds to 0 percent of oxygen and the 60 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. The low current causes 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. The display allows oxygen readings from Cat ET to be compared to dry readings from a meter such as a Teledyne meter. 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 generated 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 sensor and the oxygen buffer | |
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 P1 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. Measure the Supply Voltage for the Heater at the Buffer
- Establish communication between Cat ET and the ECM.
- Disconnect the harness connector for the oxygen sensor from the buffer.
- Connect a voltmeter between terminals A and B of the six terminal connector on the oxygen buffer. Set the voltmeter to read DC voltage.
- 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".The override changes to "OFF" only if the engine control is not in the START position.
- Measure the voltage between terminals A and B at the buffer 6-pin connector.
Expected Result:
The voltage is greater than 9 VDC.
Results:
- OK - The voltage is greater than 9 VDC. The oxygen buffer is creating the correct supply voltage for the heater of the oxygen sensor. Proceed to Test Step 3.
- Not OK - The voltage is less than 9 VDC. The oxygen buffer is not creating the correct supply voltage for the heater of the oxygen sensor. Proceed to Test Step 6.
Test Step 3. Check for Logged Diagnostic Codes
- Restore the wiring to the original configuration.
- Access the "Logged Diagnostic Codes" screen on Cat ET. If the following codes are logged, clear the codes:
- 1086-9
- 1087-3
- 1087-8
- 1088-5
- 1088-6
- 1086-9
- 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". The override changes to "OFF" only when the engine control is not in the START position.
- Access the "Logged Diagnostic Codes" screen again. Look for the following diagnostic codes again:
- 1086-9
- 1087-3
- 1087-8
- 1088-5
- 1088-6
- 1086-9
Expected Result:
At least one of the above diagnostic codes is logged.
Results:
- Logged 1086-9 - A 1086-9 diagnostic code is logged.
Repair: Perform the following steps:
- Set the engine control to the OFF/RESET position.
- Inspect the connections and the harness from the oxygen buffer to the oxygen sensor. Look for corrosion and other damage. Refer to Troubleshooting, "Inspecting Electrical Connectors".
- Make repairs, as needed. If the connectors and the harness appear to be OK, install an oxygen sensor that is known to be good. Calibrate the oxygen sensor. Refer to Troubleshooting, "Oxygen Sensor - Calibrate".
- Verify that the condition is resolved with the new oxygen sensor.
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
- Logged 1087-3 or 1087-8 Diagnostic Code - The PWM signal is noisy or the signal is missing. Proceed to Test Step 4.
- Logged 1088-5 or 1088-6 Diagnostic Code - There is a problem with the supply voltage for the oxygen buffer.
Repair: Refer to Troubleshooting, "Oxygen Sensor Buffer Supply".
STOP
- No Codes - The circuit for the oxygen sensor is OK.STOP
Test Step 4. Check the Harness for Open Circuits and Short Circuits
- Set the engine control to the OFF/RESET position.
- Disconnect the J2/P2 connectors and the three terminal connector from the oxygen buffer.
- Fabricate a jumper wire that is long enough to create a test circuit across the harness connector at the oxygen buffer. Crimp the correct connector terminals to each end of the jumper wire.
- Connect jumper wire between terminals B and C on the three terminal connector.
- Check the harness between the P2 connector and the oxygen buffer by measuring the resistance between the terminals that are listed in Table 1.
Show/hide table
Table 1 Points to Check for Continuity P2-67 Oxygen signal (PWM) P2-23 Return P2-67 Oxygen signal (PWM) Ground strap for the ECM
Expected Result:
The wiring between terminals P2-67 and P2-23 has continuity. The wiring between terminals P2-67 and the ground strap has continuity.
Results:
- OK - The wiring between terminals P2-67 and P2-23 has continuity. The wiring between terminals P2-67 and the ground strap has continuity. Proceed to Test Step 5.
- Not OK - At least one of the continuity checks is incorrect. There is a problem with the harness between the terminal box and the oxygen buffer.
Repair: Repair the harness, when possible. Replace parts, if necessary.
STOP
Test Step 5. Isolate the Oxygen Signal and Check the Signal at the ECM
- Restore the wiring to the original configuration. Connect all connectors.
- Remove the wire from terminal location P2-67 at the ECM connector.
- Use a multimeter that measures the duty cycle and the frequency of a signal. Connect one lead of the multimeter to the terminal that was removed from P2-67.
- Insert a 7X-1710 Multimeter Probe along terminal P2-23 of the ECM P2 connector. Connect the other multimeter lead to the 7X-1710 Multimeter Probe .
- 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". The override changes to "OFF" only when the engine control is not in the START position.
- Measure the duty cycle and the frequency of the signal between the wire that was removed from terminal 67 and terminal P2-23.
Expected Result:
The duty cycle of the signal is between 20 and 90 percent.
The frequency of the signal is between 375 and 625 Hz.
Results:
- OK - A valid oxygen signal is present at terminal P2-67.
Repair: Perform the following steps:
- Set the engine control to the OFF/RESET position.
- Insert terminal 67 into the P2 connector. Pull on the wire in order to verify proper installation of the terminal.
- Remove the 7X-1710 Multimeter Probe from terminal P2-23.
- Set the engine control to the COOLDOWN/STOP position. Clear all of the logged diagnostic codes.
- Turn on the oxygen buffer. Use Cat ET to check for logged 1087-03 and 1087-08 diagnostic codes.
- If either of the codes are present, replace the ECM according to Troubleshooting, "Replacing the ECM".
STOP
- Not OK - A valid oxygen signal is not present at P2-67.
Repair: Perform the following steps:
- Set the engine control to the OFF/RESET position.
- Insert terminal 67 into the P2 connector. Pull on the wire in order to verify proper installation of the terminal.
- Remove the 7X-1710 Multimeter Probe from terminal P2-23.
- Install an oxygen buffer that is known to be good.
- Verify that the condition has been resolved. If the condition is not resolved, replace the oxygen sensor. Be sure to calibrate the oxygen sensor after you replace the buffer and/or the sensor. Refer to Troubleshooting, "Oxygen Sensor - Calibrate".
STOP
Test Step 6. Check the Supply Voltage for the Oxygen Buffer
- Disconnect the three terminal connector from the oxygen buffer.
- Connect a voltmeter between terminals A and B of the three terminal connector on the oxygen buffer. Set the voltmeter to read DC voltage.
- 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". The override changes to "OFF" only when the engine control is not in the START position.
- Measure the voltage between terminals A and B on the three terminal connector.
Expected Result:
The voltage is greater than 18 VDC.
Results:
- OK - The oxygen buffer is receiving the correct supply voltage. However, the buffer is not creating the voltage for the oxygen sensor.
Repair: Perform the following procedures:
Connect the three terminal connector to the oxygen buffer. Repeat Test Step 2. If the buffer is not creating the correct voltage for the oxygen sensor, replace the oxygen buffer. Be sure to calibrate the oxygen sensor after you replace the buffer.
- Not OK - The oxygen buffer is not receiving the correct supply voltage. Proceed to Test Step 7.
Test Step 7. Check the Supply Voltage at the ECM
- Set the engine control to the OFF/RESET position.
- Remove the wires from terminal locations P2-13 and P2-23.
- Set the engine control to the COOLDOWN/STOP position.
- Measure the voltage between terminals P2-13 and P2-23.
Expected Result:
The voltage is greater than 18 VDC.
Results:
- OK - The voltage is greater than 18 VDC. The ECM is creating the correct voltage. However, the voltage is not at the oxygen buffer. There is a problem with the wiring between the ECM and the oxygen buffer.
Repair: Repair the harness, when possible. Replace parts, if necessary.
STOP
- Not OK - The voltage is less than 18 VDC. The ECM is not creating the correct voltage.
Repair: 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