Diagnostic Procedures

WARNING: This page is about a different car, the 2003 Mazda B4000, 2003 Mazda B3000, and 2003 Mazda B2300. However, it is still accessible from the selected car via links, so may be relevant.

If QUICK TEST was not performed, go to QUICK TEST under SELF-DIAGNOSTIC SYSTEM. If QUICK TEST was performed, go to next step.
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 .
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
Courtesy of MAZDA MOTORS CORP.
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.