P0156
P0156-O2 SENSOR CIRCUIT
Theory of Operation
The downstream O2 Sensor is located in the exhaust path behind the catalytic converter, is monitored for proper response to assure optimum catalytic converter efficiency. The downstream O2 response monitor is intended to diagnose a downstream O2 sensor that is not moving or stuck in a voltage window and to insure accurate information for catalyst monitor diagnosis.
- When Monitored:
The downstream oxygen sensors are monitored for faults during the one-shot test and the catalyst monitor test. The one-shot test is activated after the upstream sensor has been used for closed-loop control for 98 seconds, with battery voltage between 11.0 and 16.0 volts. If the downstream sensor switches once whilst the test is active the test will be terminated. If it does not switch, the one-shot test will be continued to allow the diagnostic tests on the downstream sensor to execute until either a DTC is reported or the one-shot test times out after 160 seconds. The catalyst monitor test is activated after engine run time of 462.5 seconds, with battery voltage between 11.0 and 16.0 volts, vehicle speed between 55 and 150 km/h (34 and 93 mph) with variation of less than 8 km/h (5 mph), engine temperature above 70° C (158° F) and evap purge active. The test runs for a maximum of 60 seconds.
- Set Condition:
The oxygen sensor signal voltage is greater than 2.20 volts for a calibrated period of time. Two Trip Fault. Three good trips to turn off the MIL.
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Pre-Diagnostic Troubleshooting Procedure.
1. ACTIVE DTC
NOTE: Check for contaminants that may have damaged the O2 Sensor: contaminated fuel, unapproved silicone, oil and coolant.
NOTE: After the repairs have been made, verify proper O2 Sensor operation. If all the O2 Sensor voltage readings have not returned to normal, follow the diagnostic procedure for the remaining O2 Sensors.
1. Start the engine.
2. Allow the engine to reach normal operating temperature.
3. With a scan tool, select View DTCs.
NOTE: It may be necessary to drive the vehicle to meet the conditions to set this DTC, try to repeat the conditions in which the fault originally set by reviewing the Freeze Frame data.
Is the DTC Active or Pending at this time?
Yes
- Go To 2
No
- Perform the INTERMITTENT CONDITION diagnostic procedure. Checking for an Intermittent DTC.
2. CHECKING THE EXHAUST SYSTEM FOR LEAKS
1. Turn the ignition off.
2. Raise the vehicle.
WARNING: The normal operating temperature of the exhaust system is very high. Never work around or attempt to service any part of the exhaust system until it has cooled. Special care should be taken when working near the catalytic converter. The temperature of the converter rises to a high level after a short period of engine operating time
3. Connect Exhaust Cone 8404-EC or 8404-EC to Air Pressure Regulator (with hose) W-18-MIL-1146AS.
CAUTION: The air pressure must not exceed 27.6 kPa (4 psi), otherwise engine damage can occur.
4. Attach shop air to the air pressure regulator.
5. Adjust the Air Pressure Regulator to 27.6 kPa (4 psi)
6. Insert the exhaust cone into the vehicle tail pipe.
7. If the vehicle is equipped with dual exhaust. Use the 8404-ECT with equipped attached plug, plug one side of the dual exhaust pipe. Pressurize the other as described above.
8. Apply SNOOP leak finder liquid or a mixture of liquid dish soap and water to the following areas:
- All welded joints from the exhaust manifold to 152.4 mm (6 inches) behind the downstream O2 sensor.
- O2 sensor seal points
- O2 sensor boss welds
- Flange/joint connection(s)
- Exhaust manifold to cylinder head connection(s)
- EGR solenoid gasket base and tube seal points (if equipped)
- Watch for the liquid/soapy water to bubble.
9. Use the following definitions to help determine if system or component repair/replacement is necessary:
Type 1 Leak is defined as a leak where very small foam like bubbles 1 mm (0.04 of an inch) or less appear.
Any Type 1 or greater leaks found in welded joints, O2 sensor seal points or O2 sensor boss welds must be repaired or the component must be replaced.
Type 2 Leak is defined as a leak where larger bubbles pea size, 8 mm (0.3 of an inch) or greater appear.
Any Type 2 or greater leaks found in flange or joint connections, exhaust manifold to cylinder head connections, or EGR gasket and tube seal points must be repaired or the components must be replaced
10. If a leak is found that matches the above definition, repair or replace the component as necessary.
11. Once the repair is complete, repeat the procedure to verify that all leaks have been repaired.
Were any exhaust leaks found?
Yes
- Repair or replace the leaking exhaust parts as necessary.
- Perform the PCM Verification Test. PCM Verification Test.
No
- Go To 3
3. (K243) O2 2/2 SIGNAL CIRCUIT
1. Turn the ignition off
2. Disconnect the 2/2 O2 Sensor harness connector.
3. Ignition on, engine not running.
4. Measure the voltage on the (K243) O2 2/2 Signal circuit in the O2 Sensor harness connector.
Is the voltage between 4.5 and 5.0 volts?
Yes
- Go To 4
No
- Check the (K243) O2 2/2 Signal circuit for a short to ground, open, or short to voltage. If OK, replace and program the Powertrain Control Module.
- Perform the PCM Verification Test. PCM Verification Test.
4. (K908) O2 RETURN DOWNSTREAM CIRCUIT
1. Measure the voltage on the (K908) O2 Return Downstream circuit in the O2 Sensor harness connector.
Is the voltage at 2.5 volts?
Yes
- Check the (K908) O2 Return Downstream circuit for a short to ground, open, or short to voltage. If OK, replace and program the Powertrain Control Module.
- Perform the PCM Verification Test. PCM Verification Test.
No
- Go To 5
5. O2 SENSOR
Repair
- Replace the O2 Sensor.
- Perform the PCM Verification Test. PCM Verification Test.