p0420 is reduced catalyst efficiency - that doesn’t always means the cat is bad, especially when you have a misfire. P0502 is the vehicle speed sensor which should have no effect on misfires.problem or take it to
So to answer your questions,
- Yes, use a pressure tap into the forward O2 sensor and run the engine open loop to check backpressure
- You have several places to insert the pressure taps, test, and decide which cat is clogged, if, in fact, they are.
- No, this is time for you to properly diagnose the problem or take it to someone who can as this car likely has several problems that need to be diagnosed and repaired one at a time.
Post back anything you’ve done to narrow down the misfire as that can force the P0420 code even with good cats. That downstream O2 is supposed to fluctuate like that, quite rapidly, that’s normal.