All,

I have been fighting with O2 sensors on and off for a year now and finally have a set in (all 4) that no longer register any of the P0171 or P0174 codes.

Now a new code is appearing, P0135. The check engine light will come on for a few days then go out again. This sequence of the check engine light going on and off has been happening now for a month. I have reset the code to make sure the same code is appearing each time, and every time the P0135 is the only code found.

Does anyone know why the check engine light would come on and go out intermittently without the codes being reset or any work whatsoever being done?

2001 Expedition, 5.4L, 226K miles

 

It would seem there is a problem with the o2 sensor heater circuit for the o2 sensor on the passenger side in the exhaust pipe before the converter.
I believe the PCM makes the ground for that heater circuit.It should get it's supply voltage from the CJB (central junction box). The supply is to both o2 heaters so I would suspect there is a poor connection in the wire to the one sensor.Trace that wire and make sure it hasn't been rubbed through somewhere or confirm a good connection from where it pairs off to the passenger side sensor.
Also check the fuse panel for any corrosion , some have a separate fuse for the heaters.

 

Mine used to do that all of the time, I never figured out what was going on because the spark plug busted out of the head before I could find the issues. Wiring maybe?