2000 Expedition 5.4 throwing codes for all 4 o2 sensors. Sensors already replaced and still throwing code

[giuliandi]

We replaced all 4 o2 sensors and immediately got a pending fault (later confirmed) again. Also engine is idling rough and has a little shake. Does someone know what this could be? No other codes but these.

 

[jr1under]

According to the OBD SysOp manual, those faults happening immediately after starting the engine indicate a voltage problem. As it's occurring on all four circuits, maybe there's a common power source to blame.

 

[Hamfisted]

You had no fault codes before you replaced the oxygen sensors right ? Sounds like a damaged harness or harness plug pins.


2000 Expedition Oxygen Sensor Wiring Diagrams



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[giuliandi]

You had no fault codes before you replaced the oxygen sensors right ? Sounds like a damaged harness or harness plug pins.


2000 Expedition Oxygen Sensor Wiring Diagrams

It had the codes before we changed them, still has them after replacement

 

[jr1under]

It had the codes before we changed them, still has them after replacement

Apparently, the culprit isn't/wasn't the sensors themselves but their power source. Hamfisted pointed out the likely suspect.

 

[pitdexion]

Fuse #23 15 amp in under hood fuse box is power source for O2 heaters. Pcm controls the grounds. It also powers evap items, trans pressure solenoid and intake runner.