But I already replaced the oxygen sensor...

This morning as I drove into work my check engine light came on. Ya know that feeling ya get when it comes on? It's not like it's gonna tell you that you have cancer or anything; but I still hate that feeling...

Before I walked into the office I did the key-turn-3-times-to-get-the-codes thingy and got the following codes:

12 - yep, got it
21 - O2 sensor or wiring faulty
21 - Why?! Two times... what does it mean?!
55 - yep, got it

BTW... '97 XJ, 4.0L, auto; Since I replaced the sensor I've only been driving the XJ for the past two weeks (less than 100 mi).

A couple months ago I replaced the oxygen sensor the LAST time it gave me a code 21. The guy at the counter asked if I wanted to replace both oxygen sensors. I only replaced the front (down pipe) oxygen sensor, but the not the cat's sensor. Heck, is that an oxygen sensor too and should I have replaced them both? Because this 2x code 21 has me scratching my head. Thanks!

Since it's a 97 you should pull the full OBDII codes. You'll get much more useful information that way. Autozone or one of those places will do it for free, just ask them.

X2, it'll tell you which O2 sensors are the faulty ones, then you can replace them instead of playing the "throw sensors till it stops throwing codes" game.

Great advice guys. I'll let ya'll know what I find out. Incidentally... what would happen if I simply ignored the check engine light assuming it was the o2 sensors? Just get bad (worse) gas milage?

You could toast your catalytic converter if it's running rich enough... I think.

I believe if it was going to damage the CAT it would throw a type A code making the light flash. I guess it might be kind of marginal damage though.

Disconnect the battery for cup of coffee or two and force the ecu back to factory settings, then try it.

Yup, post the exact code number here for best results. POXXX format.

Interpreting OBD codes is not always straight forward. ESPECIALLY when it comes to codes related to the oxygen sensor.

The code may indicate that the oxygen sensor numbers are out of spec. The actual root cause of that problem can be a number of things including a serious vacuum leak, etc. that create a condition that the oxygen sensor can't compensate for.

yeah if you drive w/o fixing it and it is running rich you will seriously reduce the life of your cat converter. If it runs really rich the cat will start glowing orange =)

As long as you get it sorted out in the next week or two i'm sure you'll be fine.

Not to beat a dead horse but this scenario is exactly why a code is almost never a green light to replace the indicated part.

The fuse box behind the battery, check the mini fuses. Its a 15amp fuse that COULD be out. That was the problem with my 02 sensors.

I have gotten the same codes from an intermittent loose connection to the Oxy. sensor.

2X on the battery disconnect. Even if it has cured it's self it takes 50 key cycles (or a bunch of miles) to clear the codes and turn off the CEL.

Codes just mean that the computer isn't getting the correct voltage from that circuit - which could be bad or corroded wiring connections, bad ground, or lastly, the O2 sensor is getting a little old and slow, not switching state from rich to lean.

UNLESS YOU TEST THE COMPONENT, YOU DON'T KNOW IT'S BAD. All you know is that the voltage is off parameter.

Nobody wants a mechanic to replace good parts, don't be that guy. Test it.

Any update on this?