LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Plymouth >> 2000 >> Neon Base, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self Diagnostics >> Diagnostic Tests >> DTC P0139: O2 Sensor 1/2 Slow Response
April 5, 2026: LEMON Manuals is launched! Read the announcement.

DTC P0139: O2 Sensor 1/2 Slow Response

NOTE: For connector terminal ID, see CONNECTOR IDENTIFICATION . For wiring diagram, see WIRING DIAGRAMS article.
NOTE: The 1/2 O2 sensor is referred to as downstream oxygen sensor in this test. Downstream oxygen sensor operation is monitored continuously for first 7 minutes of operation, after warm up, 20-60 MPH, 1150-2500 RPM and 8.2-15.7 in. Hg Manifold Absolute Pressure (MAP). DTC may be stored in Powertrain Control Module (PCM) if downstream oxygen sensor signal voltage is switching from less than .35 volt to more than .53 volt and back fewer times than required. Possible causes are: exhaust leak, slow responding downstream oxygen sensor, defective oxygen sensor heater, or defective connectors or wiring.
  1. Turn ignition on with engine off. Using scan tool, read Diagnostic Trouble Codes (DTCs). If global good trip counter is displayed and equal to zero, go to next step. If global good trip counter is not displayed or displayed with a value, go to step  5.
  2. Start engine. Check for exhaust leaks between engine and downstream oxygen sensor. If no exhaust leaks exist, go to next step. If exhaust leaks exist, repair exhaust system as necessary. Perform TEST VER-5A .
  3. Turn ignition off. Using voltmeter, check voltage drop on downstream oxygen sensor ground circuit by backprobing Black/Orange wire between downstream oxygen sensor connector terminal No. 3 and PCM connector terminal No. 27. PCM is located in engine compartment, between air cleaner and radiator. Downstream oxygen sensor is located on exhaust pipe, behind catalytic converter. Start engine and allow to idle reaching normal operating temperature. 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 the downstream oxygen sensor ground circuit (Black/Orange wire) between downstream oxygen sensor and PCM. Perform TEST VER-5A .
  4. Turn ignition off. Using voltmeter, check voltage on downstream oxygen sensor signal circuit by backprobing Tan/White wire between downstream oxygen sensor connector terminal No. 4 and PCM connector terminal No. 51. Start engine and allow to idle reaching normal operating temperature. If voltage drop is less than .1 volt, replace downstream oxygen sensor. Perform TEST VER-5A . If voltage drop is .1 volt or more, repair poor connection causing high resistance on the downstream oxygen sensor signal circuit (Tan/White wire) between downstream oxygen sensor and PCM. Perform TEST VER-5A .
  5. Start engine. Check for exhaust leaks between engine and downstream oxygen sensor. If no exhaust leaks exist, go to next step. If exhaust leaks exist, repair exhaust system as necessary. Perform TEST VER-5A .
  6. Inspect wiring and connectors between downstream oxygen sensor and PCM. Inspect related wiring harnesses and connectors. Refer to Technical Service Bulletins (TSBs) for related problems. If no problems exist, test is complete. If any problems exist, repair wiring or connectors as necessary. Perform TEST VER-5A .
scientia non olet · About LEMON Manuals