Check Engine Light

You need to check the codes, but first, you need to provide us more information on the year, make and engine of your jeep. Another cheap way is to go to one of several automotive parts businesses, since they will run the codes themselves and advise which part likely needs replacing.

Some times it's as simple as a loose gas cap. Was it running okay? Oil, water levels okay?

I checked the codes. It is 12 and 21. I have a 1997 TJ four cynlinder manual tranny. five speed.

12 Direct battery input to PCM was disconnected within the last 50 Key-on cycles.


This is code 21. Upstream oxygen sensor response slower than minimum required switching frequency. Upstream oxygen sensor heating element [COLOR=blue !important][COLOR=blue !important]circuit[/COLOR][/COLOR] malfunction. Downstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor input voltage maintained above the normal operating range. Oxygen sensor voltage too low, tested after cold start. (Upstream or Downstream) Left oxygen sensor input voltage maintained above the normal operating temperature.

Code 12 will not activate the CEL since it is only a secondary fault notice. It will display when checking for faults, so your concern is code 21. Check the wires on your o2 sensor and it is possible you may need to replace the sensor.

My manual claims this is not something that a "home mechanic" should mess with. Is that correct?

I'd rather have my hands working on my jeep than the hands of a person only concerned how much work he could pump out.

Seriously, nobody except you can determine if you are capable of this. If you feel you are not, then you might just have to bite the bullet.

I'm mechanically inclined and have done all the work so far. I haven't had to do very much engine work but I think I can handle. Why would the manual say that a licensed mechanic should do the work. Is it that difficult?

Found a youtube video on replacing the oxygen sensor. Piece of cake. Now I just have to buy a new one!

Check www.Rockauto.com I'd suggest NTK brand sensors