On my way home from work today, my check engine light came on and it shows code 21. I know this has something to do with the O2 sensor, either upstream or downstream, but could anybody give me any more specific info? I need to know what to check and how to check it.

Also I get code 12 which is the stupid 50 key turn ons since last ecu reset or whatever. The thing is, I have not reset my ecu ever. I know you can pull the neg. battery terminal or pull a fuse, but I have never done this. No one has worked on my car in the last year except for me, and when I did work on my car it was little things like oil changes. Why do I still get code 12?
Oh yeah, the battery shows a little over 12 volts.

-Jared

Hey grneclipse, if you read this, whats up with that turbo kit?

Iceman CAI | RS*R Cat-Back | AFX Pully | Milled Head 10:1 CR | MP 60mm TB

Code 12 is not unusual. Your battery voltage may have run alittle low one time and that could throw a code also. As for the 21 code, YES It is the o2 sensors. WHICH?? Probably the one on the engine but that is a guess. You need it scanned or replace both. If you have a lot of miles, think about replacing both.
GOOD LUCK

to reset the computer you need to disconnect the positive terminal, not the neg. i thought the same thing, i put on a larger throttle body and my engine light came on so i took off the negative and it didnt work, so i pulled the positive got in the car and it was off.

ASE certified master auto technician

Jdeyo,

I talked to my friend who was friends with the other guy and now it seems that he's not so sure if he wants to sell or not, however I will let you know what's up, regarding the check engine light, I just had those two codes come up on me a few days ago but it turned itself off yesterday. Regardless, I took my car to get the battery and alternator checked and sure enough, the battery was bad, sooo....I Hope this can help you, Like i said if you need to hit me up, you know where i am Jdeyo.

Don't have the stock Eclipse anymore
Just a stock Gsr now,