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 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
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. 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 a meter such as a Teledyne meter. The ECM uses the signal of the percent of oxygen for two functions:
- Make corrections for variation of fuel BTU.
- Maintain desired emissions levels.
If an incorrect signal is detected by the ECM, a diagnostic code is generated and compensation for fuel quality is disabled. 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 | g01075330 |
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Schematic of the circuit for the oxygen sensor and the oxygen buffer | |
Test Step 1. Inspect the Electrical Connectors and Wiring
- Set the engine control to the OFF/RESET mode. Switch 16 amp circuit breaker (2) to the OFF position.
Note: For the following steps, refer to Troubleshooting, "Inspecting Electrical Connectors".
- Thoroughly inspect the following connections:
- J2/P2 connectors
- J10/P10 connectors on the terminal box
- Check the torque of the allen head screw for the ECM connector. The proper torque is
6 ± 1 N·m (55 ± 9 lb in) .Show/hide table
Illustration 3 g01075280 Harness side of the P2 connector
(P2-13) +24 VDC for the buffer
(P2-23) Return
(P2-53) Return
(P2-67) Oxygen signal (PWM)Show/hide table
Illustration 4 g00894829 Harness side of the P10 connector
(P10-H) Return
(P10-K) +24 VDC for the buffer
(P10-L) Return
(P10-M) Oxygen signal (PWM) - 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 5 g01075292 (1) Oxygen sensor
(2) Connectors for the oxygen sensor
(3) Oxygen buffer
(4) P651 connector for the harness from the terminal box
(5) P652 connector for the harness to the oxygen sensor - Check the harness and wiring for abrasion and for pinch points from the oxygen sensor to the ECM.
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During operation, the oxygen sensor may reach temperatures that exceed 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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| Illustration 2 | g01075275 |
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Terminal box (1) ECM connectors J2/P2 (2) 16 amp circuit breaker (3) J10/P10 connector for the harness from the oxygen buffer | |
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 Harness Connector for the Oxygen Sensor
- Connect Cat ET to the service tool connector on the terminal box. Refer to Troubleshooting, "Electronic Service Tools".
- Disconnect the oxygen sensor from the engine harness.
- Switch the 16 amp circuit breaker ON. Set the engine control to the STOP mode.
- 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 mode.
Show/hide table
Illustration 6 g01075432 Harness connector for the oxygen sensor
(A) VH+
(B) VH- - Measure the voltage between terminals A and B on the harness connector for the oxygen buffer.
Expected Result:
The voltage is greater than 9 VDC.
Results:
- OK - The voltage is greater than 9 VDC. The correct supply voltage for the heater of the oxygen sensor is present at the harness connector. Proceed to Test Step 3.
- Not OK - The voltage is less than 9 VDC. The supply voltage for the heater of the oxygen sensor is not present at the harness connector. Proceed to Test Step 6.
Test Step 3. Check for Diagnostic Codes
- Access the "Logged Diagnostic Codes" screen on Cat ET. If the following codes are logged, clear the codes:
- 1086-09 Oxygen Sensor Element not communicating on link
- 1087-03 Oxygen Buffer Module short to +batt
- 1087-08 Oxygen Buffer Module noisy signal
- 1088-05 Oxygen Sensor Power Supply open circuit
- 1088-06 Oxygen Sensor Power Supply short to ground
- 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 mode.
- Access the "Logged Diagnostic Codes" screen again. Look for the following diagnostic codes again: 1086-09, 1087-03, 1087-08, 1088-05 and 1088-06.
Expected Result:
At least one of the above diagnostic codes are logged.
Results:
- Logged 1086-09 - A 1086-09 diagnostic code is logged.
Repair: Perform the following steps:
- Set the engine control to the OFF/RESET mode. Switch the 16 amp circuit breaker OFF.
- 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 1087-03 or 1087-08 Diagnostic Code - The PWM signal is noisy or the signal is missing. Proceed to Test Step 4.
- Logged 1088-05 or 1088-06 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 mode. Switch the 16 amp circuit breaker OFF.
- Disconnect the J2/P2 connectors and the J651/P651 connectors. Thoroughly inspect the connectors for corrosion and/or for moisture. Make repairs, if necessary.
Show/hide table
Illustration 7 g00895813 P651 harness connector for the terminal box to the oxygen buffer
(P651-B) Return
(P651-C) Oxygen signal (PWM) - Install the ends of a jumper wire into terminals P651-B and P651-C.
- Check for continuity 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 All of the remaining terminals in the P2 connector
Expected Result:
The wiring between terminals P2-67 and P2-23 has continuity. All of the other continuity checks indicate an open circuit.
Results:
- OK - The wiring between terminals P2-67 and P2-23 has continuity. All of the other continuity checks indicate an open circuit. 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
- Remove the jumper wire from the P651 connector. Reconnect the J2/P2 and J651/P651 connectors.
- Use a 151-6320 Wire Removal Tool to remove terminal 67 from the P2 connector.
- Use a multimeter that is capable of measuring 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.
- Switch the 16 amp circuit breaker ON. Set the engine control to the STOP mode.
- Access the "Diagnostic Override" screen of the 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 mode.
- Measure 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 mode.
- Reinstall the wire into terminal 67 of 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 STOP mode. 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 mode. Switch the 16 amp circuit breaker OFF.
- Reinstall the wire into terminal 67 of 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. Measure the Supply Voltage for the Heater at the Oxygen Buffer
- Set the engine control to the OFF/RESET mode. Switch the 16 amp circuit breaker OFF.
- Disconnect the P652 connector from the oxygen buffer.
- Switch the 16 amp circuit breaker ON. Set the engine control to the STOP mode.
- 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 mode.
Show/hide table
Illustration 8 g01075466 J652 connector on the oxygen buffer
(J652-A) VH+
(J652-B) VH− - Measure the voltage between terminals J652-A and J652-B on the oxygen buffer.
Expected Result:
The voltage is greater than 9 VDC.
Results:
- OK - The voltage is greater than 9 VDC. The correct supply voltage for the heater of the oxygen sensor is present at the oxygen buffer. There is a problem with the harness between the oxygen buffer and the oxygen sensor.
Repair: Repair the harness, when possible. Replace parts, if necessary.
STOP
- Not OK - The voltage is less than 9 VDC. The correct supply voltage for the heater of the oxygen sensor is not present at the oxygen buffer. Proceed to Test Step 7.
Test Step 7. Check the Supply Voltage for the Oxygen Buffer
- Set the engine control to the OFF/RESET mode. Switch the 16 amp circuit breaker OFF.
- Disconnect the P651 connector from the oxygen buffer.
- Switch the 16 amp circuit breaker ON. Set the engine control to the STOP mode.
- 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 mode.
Show/hide table
Illustration 9 g00894897 P651 harness connector for the oxygen buffer
(P651-A) +24 VDC for the buffer
(P651-B) Return - Measure the voltage between terminals P651-A and P651-B.
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:
Reconnect the J651/P651 connectors. Repeat Test Step 2. If the condition is not resolved, 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 8.
Test Step 8. Check the Supply Voltage for the Oxygen Buffer at the ECM
- Disconnect the P10 connector from the terminal box.
- Measure the voltage between terminals K and L on the terminal box's J10 connector.
Expected Result:
The voltage is greater than 18 VDC.
Results:
- OK - The voltage is greater than 18 VDC. 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
- Not OK - The voltage is less than 18 VDC. Proceed to Test Step 9.
Test Step 9. Create a Short Circuit in the Harness
- Set the engine control to the OFF/RESET mode. Switch the 16 amp circuit breaker OFF.
- Disconnect the P651 connector from the oxygen buffer.
Show/hide table
Illustration 10 g00894897 P651 harness connector for the oxygen buffer
(P651-A) +24 VDC for the buffer
(P651-B) Return - Install the ends of a jumper wire into terminals P651-A and P651-B.
- Switch the 16 amp circuit breaker ON. Set the engine control to the STOP mode.
- Access the "Logged Diagnostic Codes" screen on Cat ET. If a 1088-05 diagnostic code is logged, clear the code.
- 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 mode.
- Access the "Logged Diagnostic Codes" screen again. Look for the 1088-05 diagnostic code.
Expected Result:
The 1088-05 diagnostic code is not logged when the circuit for the supply to the buffer is shorted by the jumper wire.
Results:
- OK - The 1088-05 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 mode. Switch the 16 amp circuit breaker OFF.
- Inspect the J651 connector on the oxygen buffer for corrosion and for 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 1088-05 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 10.
Test Step 10. Check the Supply for the Oxygen Buffer at the ECM
- Set the engine control to the OFF/RESET mode. Switch the 16 amp circuit breaker OFF.
- Use a 151-6320 Wire Removal Tool to remove terminals P2-13 and P2-23.
This will create an open circuit between the buffer and the ECM.
- Switch the 16 amp circuit breaker ON. Set the engine control to the STOP mode.
- Access the "Logged Diagnostic Codes" screen on Cat ET. If a 1088-05 or 1088-06 diagnostic code is logged, clear the code.
- 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 mode.
Show/hide table
Illustration 11 g01075312 Jumper wire on the harness side of the P2 connector
(P2-13) +24 VDC for the buffer
(P2-23) Return - 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 a 1088-05 or 1088-06 diagnostic code.
Expected Result:
When an open circuit was created, a 1088-05 diagnostic code was activated.
When a short circuit was created, a 1088-06 diagnostic code was activated.
Results:
- Yes - When an open circuit was created at the P2 connector, a 1088-05 diagnostic code was activated. When a short circuit was created, a 1088-06 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 mode. Switch the 16 amp circuit breaker OFF.
Use a 151-6320 Wire Removal Tool to remove the jumper wire from the P2 connector.
Reinstall the terminals into P2-13 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 mode. Switch the 16 amp circuit breaker OFF.
Use a 151-6320 Wire Removal Tool to remove the jumper wire from the P2 connector.
Reinstall the terminals into P2-13 and P2-23. Pull on the wires in order to verify proper installation of the terminals.
It is unlikely that the ECM has failed. Exit this procedure and perform this procedure again. If the problem is not resolved, perform the following steps:
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