Help! Check engine light on!!!???

Hello,
I have a 1995 Avalon with 115,000 Km.
Well after 10 years I just replaced my battery. It just wasn't cutting it in this super cold weather anymore.
A few days passed and the CHECK ENGINE LIGHT went on.
The car runs fines. Starts great in -25c temps.
So I disconnected the battery to reset everything.
Again after a few days the CHECK ENGINE LIGHT goes on.
(I got the battery from Toyota, installed it myself.)

Is this just a coincidence and I have something else that's gone bad ??
OR
Is this battery causing the CHECK ENGINE LIGHT to go on??



Like I said, the car runs just fine.
I checked for loose or broken wires and hoses.
I also checked the gas cap.

Any ideas people??
Should I wait a while and see if the CE light goes off.
Should I rush out to the dealer to get it checked out?

TIA.

We have had a problem with the check engine light and seem to have traced the problem to the oxygen sensors... there are three and we haven't quite figured which, if any is bad. When the one in the manufold was eremoved, it was found to be loose and probably not fully installed correctly initially. We ar going to run a tap through the mounting opening and attempt to reinstall it. The other openings were fine.

The CEL is probably just a coincidence, but you'll need to find out what code has been set. You can take your car to the dealer, but they usually want to charge you somehthing just for the diagnosis. Some car parts places (like Autozone) will do it for free. Your car is OBDI, so you will have a two-digit code (like 25, or 26, etc). Can't tell you what's wrong without knowing what code was generated.