I recall reading on here that my '99 F150 5.4 has sensors before and after the cats, and that only the ones BEFORE the cats need to be serviced (changed) as the ones after only monitor the cats and don't affect engine performance. Is this true? I don't want to fix what ain't broke, but I do need to service them.

Qdude

 

That's how I understand it also. My rears are still factory, I've replaced tow sets of fronts...

 

the front ones help in air/fuel metering and do need to be changed. the rears monitor if your cats are working and dont effect the performance of your truck. at least that is what i have been told by mechanics. i would leave the rears alone until you get a check engine code that specifically says their bad.