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Home >> Jeep >> 1998 >> Cherokee Classic, 2.5 P >> Repair and Diagnosis >> External Pages >> Different car >> Section 23 (Engine Controls - Tests W/Codes) >> Self-Diagnostic Tests >> DTC P0139: 1/2 O2 Sensor Slow Response (Gas)
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DTC P0139: 1/2 O2 Sensor Slow Response (Gas)

WARNING: This page is about a different car, the 1999 Dodge Pickup and 1999 Dodge Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: For connector terminal identification, see CONNECTOR IDENTIFICATION . For circuit identification and wiring diagram, see RAM PICKUP wiring diagram in WIRING DIAGRAMS article.
NOTE: DTC P0139: 1/2 O2 SENSOR SLOW RESPONSE is monitored with engine coolant temperature more than 147°F (63.9°C), after reaching a vehicle speed of 10 MPH, and throttle remains open (off idle) for 2 minutes and subsequently bringing vehicle to a stop and allowing engine to idle in Drive (A/T) or Neutral (M/T). DTC may be stored in Powertrain Control Module (PCM) when PCM senses that 1/2 Heated Oxygen Sensor (HO2S) voltage is switching from less than .27 volt to more than .62 volt and back less times than required. Possible causes are: engine mechanical problem, exhaust leak, defective 1/2 HO2S, defective PCM, defective connectors or defective wiring.
NOTE: On 3.9L and 5.2L engines, 1/2 HO2S is located in downstream position. On 5.9L Medium-Duty (MD) and 8.0L California engines, 1/2 HO2S is located in pre-catalyst position.
  1. Start engine and allow it to idle. Check exhaust system for excessive smoke caused by coolant or oil consumption. Repair engine mechanical problem as necessary and replace 1/2 HO2S. Perform TEST VER-5A. If engine mechanical condition is okay, go to next step.
  2. Ensure engine is still idling. Check exhaust system for leaks between engine and catalytic converter. Repair exhaust system as necessary. Perform TEST VER-5A.
  3. Turn ignition off. Disconnect 1/2 HO2S connector. Clean and/or repair connector as necessary. Using an ohmmeter, check resistance between ground and 1/2 HO2S connector, sensor ground circuit (Black/Light Blue wire). If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open 1/2 HO2S sensor ground circuit. Perform TEST VER-5A.
  4. Ensure ignition is off. Ensure 1/2 HO2S connector is still disconnected. Visually inspect connector for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. Perform TEST VER-5A. If connector is okay, go to next step.
  5. Ensure ignition is off. Disconnect PCM connectors. PCM is located in engine compartment. See PCM LOCATION table under SYSTEM DIAGNOSTICS. Visually inspect connectors for corroded, damaged, pushed-out or miswired terminals. Repair connectors as necessary. Perform TEST VER-5A. If connectors are okay, go to next step.
  6. At this time, 1/2 HO2S is assumed to be defective. Replace 1/2 HO2S. Perform TEST VER-5A.
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