Hello, I am the lucky owner of a 99 zr2 blazer and I recently popped and engine code for p0420. I replaced sensors, checked for leaks, can't find any malfunctions in the cat, I do have after market flow master 40 pipe and a flow master muff. Could either of those affect the cat defficiency code?

 

if all that stuff is after the cat then it shouldnt matter. most the time a p0420 is a converter code. unless you got a scanner to check the voltages from the front and rear oxygen sensors its hard to tell if the cat itself is bad

 

I'll have to find a scanner and check it out. As far as I'm aware the cat is stock, and all after market parts are after the cat so then it's likely my cat would you say?

 

If you have one of those laser thermometers, measure the temp of the cat just right at the inlet and then measure at the just at the rear of the cat (at the outlet). Make sure you're taking the temp of the cat itself and not the heat shield. Th rear should be significantly hotter that the front, if it is working correctly. As abig34 said though, the voltages would be more accurate a diagnosis. But if the rear is colder than the front, I would look at the cat as the issue. It generally is for P0420.

 

at the other shop we worked at, we just replaced a cat at the sign of a 420 code. never diagnosed it and never had a come back. the shop im at now we actually diagnose the issue and check the oxygen sensors. but its still mostly a cat, we did have a truck in here a while back that actually had to have the computer reflashed and updated at the dealer for a software update that fixed it