P0420

03 Taco, 370K miles. Check engine came on after ignition coil went bad. I replaced bad coil p0420 still on. After driving with sound of rock tumbler under my truck for a few days the sound stopped and the check engine light disappeared a few days after that. OBD reader shows NO codes. 2 cats. I'm assuming 1 is bad but why the disappearing code act? Anybody have any suggestions?

 

One of the items that can cause an primary convertor oxygen deficiency code (P0420) is a bad coil/bad spark plug. (along with cracked exh manifolds, worn out cat, intake leaks, ...)

P0420 is the code thrown when the primary cat (one closest to the exhaust ports) does not have sufficient oxygen output as defined by a preset value in the ECU software. It's measured by the information provided to the ECU of the switching ratio frequency between the AF sensor (pre-cat) and the O2 sensor (post-cat).
There's a pretty good U-tube video that describes how it works

 

thanks for the info. I changed out the bad coil(s) but I heard the bad firing as a result can cause the cat to go. I do also have 370,000 miles on it...original cats. My plan is to change both out in the next week. The thing I'm curious about is why the check engine lite just went off after a week of it being on solid and no code is showing when i hook up the reader?

 

The p0420 code will reset itself if the condition is no longer happening, which then resets the MIL (ck eng lite).
wow! 370 K on a cat is a good thing.

Ii wouldn't change the cats unless you have proof (gas reading from an analyzer) they're not working first. they can be expensive.