P1129
Downstream Oxygen Sensors Swapped Bank to Bank
Findings, check these first
- Rear sensor connectors crossed during exhaust service
- Catalytic converter replacement with harnesses mixed
- Identical rear connectors allowing a swap
- Harness clipped to the wrong side
- Prior repair leftovers
Standard procedure
- Trace each downstream connector to its bank
- Correct the routing and reseat the connectors
- Clear codes and run the catalyst monitor
- Verify each rear sensor follows its own bank
- Label connectors for future service
Code lookup
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Sources and review
Review: qualified. Read the source limits below.
- OBDII.pro: P1129 Ford definition
live reference page (obdii.pro)
Supports: Ford-family meaning for P1129: Downstream Heated O2 Sensors Swapped
Fetched directly. obdii.pro is a publisher independent of the engine-codes family.
Last updated 2026-09-05.
Report a correction for P1129. Include your vehicle, the issue and a source if you have one.
This fault is part of the Ford fuel and ignition codes guide. It covers the family and the order to check it.
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