What causes the cat to clog?. How many cats are on a 4.0 Explorer. How hard of a job it is for a shadetree. Local mechanic told wife that the cats are clogged and wants 500 to replace (2) cats, needless to say I'm leary.

 

The engine running rich will clog cats.
From what I've gathered, Ford uses two cats. One reduces hydrocarbon emissions, the other reduces NOx emissions. Most aftermarket cats combine both operations into one converter.

 

What year is it? I work at Ford in Hazelwood Missouri and build 'em. Right now we have one on each side of the y-pipe. It's changed as one piece. Drop the crossmember, 2 nuts to each side manifold, 2 spring bolts to the muffler. Pull 4 HEGO wires and change. With a new vehicle, a hoist and all the right tools, I can do it in about an hour.

 

It's a 99 EXP with a 4.0 sohc. is there any welding or cutting that has to be done?

 

Offhand I can't exactly remember the '99. If it's like they are now you would just replace the y-pipe assembly. There would be no cutting or welding unless it's REALLY rusted. If you replace just the converter, you would have to cut it out and weld or clamp a new one in.