Hello All!


I have a 99 v70-xc awd w/ 138K miles. My check engine light has been on & off most of the past 2 years; mostly on. Last summer, this car passed North Carolina inspection test with the problem present. I have no idea how???

Recently, I left the car w/ my indy mechanic for a few weeks. He [the mechanic] said it's giving multiple codes: P0420, P0421 and P0422. He hadn't driven the car for a few days & found it had a dead battery - which he said could have been contributing to the varying codes. He replaced the battery, reset the codes and drove the car again until the check engine light came on again. I think the code was P0422. He told me it didn't need a new cat - an O2 sensor would resolve my problem. So, the mechanic installed a new front O2 sensor. He did not install a new back sensor. I drove the car for about 15 miles and the check engine light came on again. This time the code was definitely P0422. I called back the mechanic. He thinks the cat is causing this problem.

Money is tight for us and I'm trying to sell this car. I don't have money to experiment with the cat verses back O2 sensor. I really want the problem resolved so I can sell this car and feel good the new owner has a sound vehicle which I represented honestly.

Is it the cat or the back O2 sensor?

How can I determine what the problem really is without a process of elimination?


Thank you in advance for all replies!
Robin