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

If your tech thinks a dead or weak battery can contribute to cat or O2 sensor codes he doesn't have much understanding of the OBDII system. If someone else is available they might be a better bet. At least start over with whoever you choose and get a correct reading of which codes are actually set, then look them up.

P0420 is a catalyst efficiency code and can be caused by a bad cat if nothing else is wrong. It may show up (with other codes too) if the engine is running incorrectly, e.g. too rich.

If you receive codes related to either O2 sensor they are usually more specific and will point to which sensor.

However as Klaus will tell you various vacuum or exhaust leaks can cause O2 or cat related sensor codes so those should be eliminated systematically first.

good luck!

The front and rear O2 sensors work together, the rear sensor determines how good the front O2 sensor is working. I wonder if the front O2 sensor is a Bosh and if it is wired up correctly. I doubt the cat converter is bad, that is not one of the messages, but it is always a possibility.
Have the shop check the voltage of the front O2 sensor to ensure it is working properly. Also check for vacuum leaks that would make your engine run rich. They can also check the rear O2 voltage to make sure it is fluctuating correctly.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)