My 1996 legacy wagon 2.2 Auto. Check engine light has been going on and off for a few months . I have been able to turn it off by unhooking the battery for 20 minuets but it been comming back. Autozone put a code reader on it and it showed P0420- catalyst efficency below threshold(Bank 1).
What does this mean and what do I have to fix.

Well I can tell you what it means, but where to go from there you have a few options.

Basically you have 2 oxygen sensors. One before the cat, one after. The first gets a reading of the air, then teh air passes through the cat converter, then the 2nd gets a reading of the air. These numbers should be different if the cat is functioning properly. This code is telling you that the cat isn't working optimally.

A few suggestions/thoughts:

1) obvious solution, replace the cat...but this can be costly (they aren't cheap) and this may not be the exact problem. There maybe another problem that is causing your cat problems.

2) when you say you've been getting this off and on, have you been disconnecting your battery and reseting your ECU to clear it, or has it been doing this on its own?
If it's been doing it on it's own, I'd disconnect the negative cable ont eh battery for 15 minutes, this will clear the ECU (car's computer) and allow it to reset itself and 'relearn'. If it comes back, you have a problem.

Are you losing oil or coolant? Do you have smoke coming out of your tailpipe? If so is it blue, black, or white? Is your gas mileage suffering?

3) I've heard something about taking cars to dealerships to have their ECU's reprogrammed to 'widen' the values they will accept from the o2 sensors. I really don't know anything about this, so I can't tell you if this is bogus, or even possible with your car, or what...just something I've heard mentioned.

So yeah, my suggestion would be to check fluilds and for smoke, possibly something is leaking and getting burnt in your exhaust, causing your cat damage...if so this needs to be addressed prior to putting in a new cat.
Then reset your ECU and drive on it for awhile and see if it comes back on.
Keep us updated so we can help.

I bought my 1999 subaru legacy 2.2 off of ebay, replaced the tranny, replaced both O2 sensors, replaced the TPS sensor, and now the last error left is the P0420 as you mentioned. I now have to ask... did you resolve your issue? if so, what was the answer?

I have until april to get the car inspected by New York State. Its been too cold up here to replace the O2 sensor. There is a chance that you may have to replace the catlatic converter. good luck.

Himie