My grandparents check engine light came on in the 1997 Explorer. It has the 5.0, 4x4, auto. Took it to autozone and code P0420 came up.
Any ideas what could be the problem?

 

P0420 is "Cat convertor efficiency below threshold bank one" which is passenger side.

Since thats the only code that came up the cat most likely needs to be replaced.

Best thing to do is take it to a GOOD independant shop & have it checked out.

BTW, it looks like this is past the 8 year/80K federal emmisions warranty, but maybe not.

If the truck is less than 8 years old & has less than 80K miles, check with your Ford dealer. Might get a free replacement cat.

 

Looks like there is a TSB on the PO430/0420. Check out the post from projectsho89 in the thread "97 Mercury Mountaineer PO430 emissions error."

 

Check out this thread https://www.ford-trucks.com/forums/s...d.php?t=440835

For some reason everyone is quick to think that the cat is bad, others think that replacing the downstream (after cat) O2 sensors will help, it won't. I replaced the upstream sensors (before cat) and the code and light were gone after a few driving cycles, it has been code free for two months now.

Good luck