HELP! anyone!... i have a 1999 miata. i just replaced the 02sensor (rear) and the ignition coil. can a bad catalytic converter damage the new sensor i put in. im not sure if the cat is bad. the check engine light went off from a p0421. (since i replaced the 02 sensor) Also, would the check engine light go on if i had a bad cat?

Thanks!

 

It's possible. Did you have the exhaust manifold checked? My 1999 light came on; my independent worked on it for four hours without success, and then I took it to my local dealer. They diagnosed in 15 minutes, ran cleaner through the exhaust system, light went out and hasn't been back on since. This was six months ago. Just had the car smogged for registration and passed with flying colors.

 

The dealer told me to check the voltage on both sensors. the front sensor should change faster than the rear and if it doesn't then thats a clear indicator the Cat. is bad. i did check the sensors, and the rear does change dramatically faster. im in the process of purchasing a new Cat. (which in California you can only buy it through the dealer). im kinda gambling that its the cat, but everything seems to lead that way. im going to put it in myself. does anyone have any helpful hints on putting a new CAT in? Its a direct fit (bolts right in) and doesn't seem to difficult.

thanks!