My 98 SOHC is throwing 12, 21, and 37. I know what they mean but I'm not sure what to do. How do I find out if my O2 sensor is bad before or after my CAT? I need some help here guys. ALso when I turn the wheel all the way over it idles down and sometimes dies. It's really making me mad.

 

Take the car to an Autozone or O'Reillys and have them scan it for the "P" code. That will narrow it down to which sensor. Most likely, it's the upstream. Inspect the wiring.