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Diagnostic Procedure

WARNING: This page is about a different car, the 2002 Isuzu Trooper. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: Engine And HO2S must be cool for at least 15 minutes before this test can be performed.
  1. Perform ON-BOARD DIAGNOSTIC II SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM. After performing OBD-II system check, go to next step.
  2. Allow engine to cool. Disconnect Fuel Pump (FP) relay. FP relay is located in relay box next to battery. Connect fused jumper between FP relay socket cavities No. 2 (Red wire) and No. 4 (White wire). See Fig 1. Turn ignition on. Using scan tool, observe Bank 1 HO2S No. 1 parameter for several minutes. If HO2S parameter changes from bias (400-500 millivolts) to 650 millivolts or more, or less than 250 millivolts, check for an intermittent problem. See DIAGNOSTIC AIDS . If parameter does not change as specified, go to next step.
  3. Inspect O2 SENS HEATER fuse (20-amp). Fuse is located in engine compartment fuse box next to battery. If fuse is blown, go to step  13. If fuse is okay, go to next step.
  4. Turn ignition off. Raise and support vehicle. Disconnect Bank 1 HO2S No. 1 4-pin harness connector. Bank 1 HO2S No. 1 is located in right exhaust pipe. Turn ignition on. Using a test light connected to ground, probe Bank 1 HO2S No. 1 harness connector No. 4 (Red/White wire). See Figure. If test light illuminates, leave ignition on and go to next step. If test light does not illuminate, go to step  7.
  5. Connect test light between Bank 1 HO2S No. 1 harness connector No. 3 (Black wire) and No. 4 (Red/White wire). If test light illuminates, go to next step. If test light does not illuminate, go to step  8.
  6. Turn ignition off. Allow HO2S to cool for at least 15 minutes. Measure resistance between Bank 1 HO2S No. 1 pigtail connector terminal No. 3 and 4 (both wires are Black). Use reverse view of Figure. Resistance should be 3-6 ohms. If resistance is as specified, go to step  9. If resistance is not as specified, go to step  10.
  7. Repair open in Bank 1 HO2S No. 1 ignition feed circuit, Red/White wire between OS SENSOR HEATER fuse and HO2S. After repair is complete, see VERIFYING VEHICLE REPAIR  under SELF-DIAGNOSTIC SYSTEM.
  8. Repair open in Bank 1 HO2S No. 1 ground circuit, Black wire between ground and HO2S. HO2S are grounded through PCM. See WIRING DIAGRAMS article.
  9. Check for poor, loose or corroded terminals in Bank 1 HO2S No. 1 harness connector. If problem is found, repair as necessary. After repair is complete, see VERIFYING VEHICLE REPAIR  under SELF-DIAGNOSTIC SYSTEM. If no problem is found, go to next step.
  10. Turn ignition off. Disconnect PCM 80-pin harness connectors E-21 (Blue) and E-22 (Red). PCM is located at right rear of engine compartment. Measure resistance of Bank 1 HO2S No. 1 high (Red wire) and low (Green wire) signal circuits between PCM harness connector and Bank 1 HO2S No. 1 harness connector. See WIRING DIAGRAMS article. If resistance in either circuit is 5 ohms or more, repair open circuit or poor connection as necessary. After repair is complete, see VERIFYING VEHICLE REPAIR  under SELF-DIAGNOSTIC SYSTEM. If no problem with wiring is found, go to next step.
  11. Check for poor, loose or corroded terminals at PCM harness connector E-22 terminals No. 18 (Green wire) and No. 58 (Red wire). If problem is found, repair as necessary. If no problem is found, go to next step.
  12. Replace Bank 1 HO2S No. 1. Before replacing sensor, verify that cause of sensor failure has been corrected (fuel contamination, improper use of RTV sealant, excessive engine oil consumption, etc.) After repair is complete, see VERIFYING VEHICLE REPAIR  under SELF-DIAGNOSTIC SYSTEM.
  13. Locate and repair short in Bank 1 HO2S No. 1 ignition feed circuit (Red/White wire) between FP relay and O2 SENSOR HEATER fuse and replace fuse. After repair is complete, see VERIFYING VEHICLE REPAIR  under SELF-DIAGNOSTIC SYSTEM.
Fig 1: Identifying Fuel Pump Relay Socket Cavities
G00077226Courtesy of ISUZU MOTOR CO.
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