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DTC P0139: 1/2 O2 Sensor Slow Response: Testing

NOTE: 1/2 HO2S may also be referred to as a downstream oxygen sensor. On 2.5L and 4.0L Federal models, 1/2 HO2S is located behind catalytic converter. On 4.0L Calif. models, 1/2 HO2S is located in front exhaust pipe below front mini-catalytic converter.
  1. Using scan tool, read DTCs. If DTC SPECIFIC GOOD TRIP counter is displayed and displayed count is "0", go to step  5 . If DTC SPECIFIC GOOD TRIP counter is not displayed or displayed count is not "0", go to next step.
  2. Turn ignition on, with engine off. Using a voltmeter, backprobe Brown/Yellow wire between 1/2 HO2S connector and terminal No. 4 at PCM C1 connector. See Figure . Wiggle wiring harness between PCM and 1/2 HO2S while monitoring voltage drop. If voltage drop is less than .1 volt while wiggling wiring harness and connectors, go to next step. If voltage drop is .1 volt or more while wiggling wiring harness and connectors, repair poor connection causing high resistance on 1/2 HO2S ground circuit (Brown/Yellow wire) between 1/2 HO2S and PCM.
  3. Ensure ignition is still on, with engine off. Using voltmeter, backprobe Tan/White wire between 1/2 HO2S and terminal No. 25 at PCM C1 connector. See Figure . Wiggle wiring harness between PCM and 1/2 HO2S while monitoring voltage drop. If voltage drop is less than .1 volt while wiggling wiring harness and connectors, go to next step. If voltage drop is .1 volt or more while wiggling wiring harness and connectors, repair poor connection causing high resistance on 1/2 HO2S signal circuit (Tan/White wire) between 1/2 HO2S and PCM.
  4. At this time, conditions for 1/2 HO2S slow response do not exist or fault is an intermittent problem. Using scan tool, read FREEZE FRAME data while attempting to duplicate the condition that caused DTC to be set by paying special attention to the conditions that 1/2 HO2S is monitored. 1/2 HO2S is monitored with engine temperature more than 147°F (64°C), after reaching vehicle speed of 10 MPH, throttle is off idle for 2 minutes, vehicle is brought to stop and allowed to idle in Drive (A/T) or Neutral (M/T). If 1/2 HO2S slow response reoccurs, go to next step. If HO2S slow response does not reoccur, condition no longer exists. Test is complete.
  5. Start engine and allow it to idle. Check exhaust system for excessive smoke caused by internal coolant or oil consumption. Repair engine mechanical problem as necessary and replace 1/2 HO2S. If engine mechanical condition is okay, go to next step.
  6. Ensure engine is still idling. Check exhaust system for leaks between engine and catalytic converter. Repair exhaust system leaks as necessary. If exhaust system is okay, go to next step.
  7. Turn engine off. Turn ignition on, with engine off. Using a voltmeter, backprobe Tan/White wire between 1/2 HO2S connector and terminal No. 25 at PCM C1 connector. If voltage drop is less than .1 volt, go to next step. If voltage drop is .1 volt or more, repair poor connection causing high resistance on 1/2 HO2S signal circuit (Tan/White wire) between 1/2 HO2S and PCM.
  8. Ensure ignition is still on, with engine off. Using voltmeter, backprobe Brown/Yellow wire between 1/2 HO2S and terminal No. 4 at PCM C1 connector. If voltage drop is less than .1 volt, replace 1/2 HO2S. If voltage drop is .1 volt or more, repair poor connection causing high resistance on 1/2 HO2S ground circuit (Brown/Yellow wire) between 1/2 HO2S and PCM.
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