P0157-O2 SENSOR 2/2 CIRCUIT LOW
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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 compares the oxygen content of the surrounding air with 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 heat the sensor to bring the sensor up 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.
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When Monitored:
Engine running for less than 30 seconds and the O2 Sensor Heater Temperature is less than 251°C (484°F) with battery voltage greater 10.99 Volts.
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Set Condition:
The Powertrain Control Module (PCM) detects that the Oxygen Sensor 2/2 signal voltage is below the minimum acceptable value. The DTC will set as Pending after one trip and Active after two trips. Three good trips to turn off the MIL.
Possible Causes |
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| (K904) O2 SENSOR 2/2 RETURN CIRCUIT SHORTED TO GROUND |
| (K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO GROUND |
| (K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO THE (K904) O2 SENSOR 2/2 RETURN CIRCUIT |
| (K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO THE (Z902) O2 SENSOR 2/2 HEATER GROUND CIRCUIT |
| O2 SENSOR 2/2 |
| POWERTRAIN CONTROL MODULE (PCM) |
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
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 allow it to idle for at least 60 seconds. |
When the engine is operating, do
not stand in direct line with the fan. Do not put your hands near
the pulleys, belts or fan. Do not wear loose clothing. Failure to
follow these instructions may result in possible serious or fatal
injury.
WARNING:
| 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. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
2. (K243) O2 SENSOR 2/2 SIGNAL CIRCUIT
| 1. | Turn the ignition off. |
| 2. | Disconnect the O2 Sensor 2/2 Sensor harness connector. |
| 3. | Ignition on, engine not running. |
| 4. | With the scan tool, monitor the O2 Sensor 2/2 Sensor voltage. |
Is the O2 Sensor 2/2 voltage between 4.1 and 5.0 Volts?
Yes
- Go To 3
No
- Go To 5
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| 1. | Measure the voltage on the (K904) O2 Sensor 2/2 Return circuit in the O2 Sensor 2/2 harness connector. |
Is the voltage at 2.5 Volts?
Yes
- Verify that there is good pin to terminal contact in the O2 Sensor 2/2 and Powertrain Control Module connectors. If OK, replace the O2 Sensor 2/2. (Refer to 14 - Fuel System/Fuel Injection/SENSOR, Oxygen - Removal) .
- Perform the POWERTRAIN VERIFICATION TEST. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
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Go To 4
4. (K904) O2 SENSOR 2/2 RETURN CIRCUIT SHORTED TO GROUND
 |
| 1. | Turn the ignition off. |
| 2. | Disconnect the PCM C2 harness connector. |
| 3. | Measure the resistance between ground and the (K904) O2 Sensor 2/2 Return circuit in the O2 Sensor 2/2 harness connector. |
Is the resistance below 100 Ohms?
Yes
- Repair the short to ground in the (K904) O2 Sensor 2/2 Return circuit.
- Perform the POWERTRAIN VERIFICATION TEST. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
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Go To 8
5. (K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO GROUND
 |
| 1. | Turn the ignition off. |
| 2. | Disconnect the PCM C2 harness connector. |
| 3. | Measure the resistance between ground and the (K243) O2 Sensor 2/2 Signal circuit in the O2 Sensor 2/2 harness connector. |
Is the resistance below 100 Ohms?
Yes
- Repair the short to ground in the (K243) O2 Sensor 2/2 Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
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Go To 6
6. (K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO THE (K904) O2 SENSOR 2/2 RETURN CIRCUIT
 |
| 1. | Measure the resistance between the (K243) O2 Sensor 2/2 Signal circuit and the (K904) O2 Sensor 2/2 Return circuit in the O2 Sensor 2/2 harness connector. |
Is the resistance below 100 Ohms?
Yes
- Repair the short between the (K904) O2 Sensor 2/2 Return circuit and the (K243) O2 Sensor 2/2 Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
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Go To 7
7. (K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO THE (Z902) O2 SENSOR 2/2 HEATER GROUND CIRCUIT
 |
| 1. | Measure the resistance between the (K243) O2 Sensor 2/2 Signal circuit and the (Z902) O2 Sensor 2/2 Heater ground circuit in the O2 Sensor 2/2 harness connector. |
Is the resistance below 100 Ohms?
Yes
- Repair the short between the (Z902) O2 Sensor 2/2 Heater ground circuit and the (K243) O2 Sensor 2/2 Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
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Go To 8
8. POWERTRAIN CONTROL MODULE (PCM)
| 1. | Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the Oxygen Sensor 2/2 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 Oxygen Sensor 2/2 and Powertrain Control Module connectors. |
| 4. | Perform any Technical Service Bulletins that may apply. |
Were there any problems found?
Yes
- Repair as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
- Replace and program the Powertrain Control Module in accordance with the Service Information. (Refer to 08 - Electrical/8E - Electronic Control Modules/MODULE, Powertrain Control - Removal) .
- Perform the POWERTRAIN VERIFICATION TEST. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).