Possible causes

Crossed HO2S harness connectors (downstream)
Crossed HO2S wiring at the harness connectors (downstream)
Crossed HO2S wiring at the 104-pin harness connectors (downstream)

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P1129 FORD Description

The heated oxygen sensor (HO2S) monitor determines if the HO2S signal response for a fuel shift corresponds to the correct engine bank. The test fails when a response from the HO2S being tested is not indicated.

FORD Technical Service Bulletins for P1129 code