P0138
REAR HEATED OXYGEN SENSOR (OXYGEN SENSOR 2)
Fault Symptom.
CHECK ENGINE lamp (MIL) on.
Any other diagnostic trouble codes, such as misfiring and adaptation.
Conditions.
Sensor preheating active more than 200 seconds after starting
and
P0136
Sensor 2 voltage = 0 V for 200 seconds.
P0137
Shorting to ground for more than 0.12 seconds,
P0138
Shorting to battery positive (B+) for more than 0.12 seconds.
P0139
Sensor 1 voltage oscillates at too low a frequency.
P0140
No voltage from the sensor, open circuit for more than 100 seconds.
Diagnostic Help.
The performance of the oxygen sensor can be ascertained with an ISAT scan tool.
- Select "READ FUNCTIONS".
- Select "OXYGEN SENSOR".
- Read "OXYGEN SENSOR 2".
At full load the ISAT scan tool should show a reading that is > 0.7 V.
At fuel shut-of the ISAT scan tool should show a reading of about 0 V.
NOTE:
^ If the reference ground or sensor voltage of any of the oxygen sensors is shorted to battery negative (B-), diagnostic trouble codes P0131 and P0137 could be generated simultaneously.
^ If the reference ground or sensor voltage of any of the oxygen sensors is shorted to battery positive (B+), diagnostic trouble codes P0131 and P0138 could be generated simultaneously.
^ If battery positive voltage (B+) is applied to any reference ground, the control module could be damaged.
If the preheating element of an oxygen sensor develops a fault the performance of the sensor will deteriorate, particularly at idling speed. A diagnostic trouble code indicating a defective sensor could then be generated. If a diagnostic trouble code for oxygen sensor preheating is generated, it should be attended to first. Following this, the performance of the oxygen sensor should be checked.
Check the Wiring
Intermittent faults may occur as a result of temporary short circuits and breaks in the wiring. Jiggle the wiring at several points and in different directions to ascertain whether the wiring harness, including any connectors, is causing the trouble. Observe the multimeter, ISAT scan tool or test lamp while doing this.
Diagnostic Procedure.
1 Check other Motronic 4.1 diagnostic trouble codes
- Obtain readouts of all diagnostic trouble codes using the ISAT scan tool.
Any other diagnostic trouble codes?
YES:
^ Continue fault diagnosis for the relevant diagnostic trouble code.
NO:
^ Continue with point 2.
2 Check the oxygen sensor's control module input
- Unplug the oxygen sensor's 4-pin connector.
- Connect an ISAT scan tool.
- Turn the ignition switch to the ON position.
- Select "READ FUNCTIONS".
- Select "OXYGEN SENSOR".
- Read "OXYGEN SENSOR 2".
The ISAT scan tool should show a reading of about 0.45 V.
- Connect a jumper lead between pins 3 and 4 in the oxygen sensor's connector.
The ISAT scan tool should show a reading of 0 V.
Are all readings OK?
YES:
^ Change the oxygen sensor. Continue with point 4.
NO:
^ Check the relevant lead and repair or replace it if necessary. Continue with point 3.
3 Check The Wiring
- Check for continuity between:
^ control module pin 11 and connector pin 3.
If the ISAT scan tool shows the two diagnostic trouble codes P0137 and P0131, the front heated oxygen sensor's ground connection should also be checked.
- Check for continuity between:
^ control module pin 40 and connector pin 3.
Is the wiring OK?
YES:
^ Continue with point 4.
NO:
^ Repair or replace the wiring. Continue with point 4.
4 Final check
- Clear the diagnostic trouble code.
- Implementation of driving cycle:
^ Drive the car at varying engine loads and rpm for 5 minutes.
- Evaluation of driving cycle:
^ Connect an ISAT scan tool.
^ Select "ON/OFF.
^ Select "DIAGNOSIS STATUS".
^ Select "OXYGEN SENSOR".
^ Read "OXYGEN SENSOR 2".
Does the ISAT scan tool show "NOT READY"?
YES:
^ Repeat the driving cycle.
NO:
^ Check whether the diagnostic trouble code has recurred.
Has the diagnostic trouble code recurred?
YES:
^ Continue at "Before Replacing a Control Module". Component Tests and General Diagnostics
NO:
^ The steps taken to rectify the fault were correct.