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Home >> Mazda >> 2003 >> B4000 Base, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P1128: Front Ho2S Signal >> Diagnostic Procedures
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Diagnostic Procedures

  1. If QUICK TEST was not performed, go to QUICK TEST  under SELF-DIAGNOSTIC SYSTEM. If QUICK TEST was performed, go to next step.
  2. KOER DTC P1128 -  Inspect front HO2S 4-pin harness connectors for indication of crossed circuits (stretched wires and wire harness not properly mounted). If connectors are okay, go to next step. If connectors are crossed, reconnect connectors correctly to HO2Ss. After repair, go to step  4 .
  3. Verify Proper HO2S SIG Pin Location -  Disconnect PCM 104-pin harness connector. Connect Breakout Box (49-UN01-130) to PCM harness connector only. See Figure . Disconnect both front HO2S 4-pin harness connectors. Measure resistance between appropriate breakout box terminals and appropriate HO2S 4-pin harness connector:
    • HO2S11 (Front Right), breakout box terminal No. 60.
    • HO2S21 (Front Left), breakout box terminal No. 87.
    See Fig 1 . If resistance reading is less than 5 ohms, fault cannot be duplicated at this time. See INTERMITTENT  testing under SYSTEM TESTS. If resistance reading is 5 ohms or more, go to next step.
    Fig 1: HO2S Harness Connector Circuit Identification
    G95E35968Courtesy of MAZDA MOTORS CORP.
  4. Verify Trouble Shooting Of DTCs Completed -  Reconnect all electrical connectors. Clear DTC from PCM memory. Perform QUICK TEST  under SELF-DIAGNOSTIC SYSTEM. If same DTC is present, replace faulty PCM. Repeat QUICK TEST  under SELF-DIAGNOSTIC SYSTEM. If any other DTC is present, go to applicable procedure for other DTCs. See DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no DTC is present, trouble shooting is complete.
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