I failed emissions with p0421 and a misfire code ( don't remember specific one). I changed spark plugs and both 02 sensora and the misfire code went away but I still have the p0421.

Torque app shows the voltage of rear 02 sensor fluctuating approximately the same as the front sensor. From what I read, this indicates a bad cat. However, I also read p0421 can be thrown bc a bad hose or other reason.

My old man says I should take it to the dealer for a diagnostics. But I don't want to pay $115 for them to just give me the obd code and the listed causes for the obd code. I'd rather that obd code go towards the cat.

If I take it to a dealer for a diagnostics, will they give me more than the obd code and the listed reasons for that obd code?

 

Do you have the stock exhaust manifold on the car? If so, the cat in the manifold has probably failed. You're probably better off replacing the manifold with an aftermarket one. Then using a set of antifoulers on the 2nd o2 sensor to prevent a code from tripping.