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P0137






P0137-O2 SENSOR 1/2 CIRCUIT LOW

For a complete wiring diagram, refer to the Wiring Information.





Theory of Operation

The Oxygen Sensors (O2 Sensor) are used for fuel control and catalyst monitoring. Each O2 Sensor measures the oxygen content of the exhaust stream. When the engine is started, the Powertrain Control Module (PCM) operates in an Open Loop mode, ignoring the O2 Sensor signal voltage while calculating the air-to-fuel ratio. The heating elements inside each O2 Sensor heats the sensor to bring it to operating conditions faster. This allows the system to enter Closed Loop earlier and the PCM to calculate the air-to-fuel ratio sooner. While the engine runs, the O2 Sensor heats up and begins to generate a voltage within a range of 0-1,275 mV. Once sufficient O2 Sensor voltage fluctuation is observed by the PCM, Closed Loop is entered. The PCM uses the O2 Sensor voltage to determine the air-to-fuel ratio. An O2 Sensor voltage that increases toward 1,000 mV indicates a rich fuel mixture. An O2 Sensor voltage that decreases toward 0 mV indicates a lean fuel mixture. The Powertrain Control Module (PCM) makes short term and long term fuel corrections to maintain stoichiometric fuel/air ratio for best catalytic converter efficiency. Short term fuel correction is based on upstream O2 Sensor output and is designed for quick engine response. The long term fuel correction compensated for variations in the engine specifications, sensor tolerances and component aging and is designed to correct rich and lean conditions over a longer period of time.

- When Monitored:
Engine running and the O2 Sensor is in closed loop mode

- Set Condition:
The Powertrain Control Module (PCM) detects that the O2 Sensor 1/2 signal voltage is below the minimum calibrated threshold. The DTC will set as Pending after one trip and Active after two trips. Three good trips to turn off the MIL.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Pre-Diagnostic Troubleshooting Procedure.

Diagnostic Test

1. ACTIVE DIAGNOSTIC TROUBLE CODE (DTC)
1. Turn the ignition on.
2. With the scan tool, read DTCs. Record DTC and Freeze Frame information.
3. Start the engine and test drive the vehicle long enough for the O2 Sensor to enter closed loop operation.
4. With the scan tool, read the active DTCs.

Is the DTC Active at this time?

Yes

- Go To 2

No

- Perform the INTERMITTENT CONDITION diagnostic procedure. Intermittent Condition Test.

2. (K904) O2 SENSOR 1/2 RETURN CIRCUIT SHORTED TO GROUND
1. Turn the ignition off.
2. Disconnect the O2 Sensor 1/2 harness connectors.
3. Disconnect the PCM C1 harness connector.
4. Measure the resistance between ground and the (K904) O2 Sensor 1/2 Return circuit at the O2 Sensor 1/2 harness connector.

Is the resistance below 10k Ohms?

Yes

- Repair the (K904) O2 Sensor 1/2 Return circuit for a short to ground.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

No

- Go To 3

3. (K141) O2 SENSOR 1/2 SIGNAL CIRCUIT SHORTED TO GROUND
1. Measure the resistance between ground and the (K141) O2 Sensor 1/2 Signal circuit at the O2 Sensor 1/2 harness connector.

Is the resistance below 10k Ohms?

Yes

- Repair the (K141) O2 Sensor 1/2 Signal circuit for a short to ground.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

No

- Go To 4

4. (K141) O2 SENSOR 1/2 SIGNAL CIRCUIT SHORTED TO THE (K904) O2 SENSOR RETURN CIRCUIT
1. Measure the resistance between the (K141) O2 Sensor 1/2 Signal circuit and the (K904) O2 Sensor 1/2 Return circuit at the O2 Sensor harness connector.

Is the resistance below 10k Ohms?

Yes

- Repair the short between the (K141) O2 Sensor 1/2 Signal circuit and the (K904) O2 Sensor 1/2 Return circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

No

- Go To 5

5. (K141) O2 SENSOR 1/2 SIGNAL CIRCUIT SHORTED TO THE (K299) O2 SENSOR 1/2 HEATER CONTROL CIRCUIT
1. Measure the resistance between the (K141) O2 Sensor 1/2 Signal circuit and the (K299) O2 Sensor 1/2 Heater Control circuit at the O2 Sensor 1/2 harness connector.

Is the resistance below 10k Ohms?

Yes

- Repair the short between the (K141) O2 Sensor 1/2 Signal circuit and the (K299) O2 Sensor 1/2 Heater Control circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

No

- Go To 6

6. O2 SENSOR
1. Replace the O2 Sensor 1/2.
2. Reconnect the PCM C1 harness connector.
3. Turn the ignition on.
4. With the scan tool, erase DTCs.
5. Start the engine and test drive the vehicle long enough for the O2 Sensor to enter closed loop operation.
6. With the scan tool, read the active DTCs.

Did the DTC return as active or pending?

Yes

- Go To 7

No

- Repair is complete.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

7. POWERTRAIN CONTROL MODULE (PCM)
1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the Oxygen Sensor 1/2 and the Powertrain Control Module (PCM).
2. Look for any chafed, pierced, pinched or partially broken wires.
3. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the O2 Sensor and Powertrain Control Module harness connectors.
4. Perform any Technical Service Bulletins that may apply.

Were there any problems found?

Yes

- Repair as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

No

- Replace and program the Powertrain Control Module. .
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

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