1994 Jeep Cherokee Engine stuttering

OK the diagnostic port is located just under the dash use pic below. Now this code reader that your friend has must be able to read OBD1. If it can't then call around to your local parts store's and see if one of them offer a free scan and if so have them scan it. Get back to me with the codes and we will see what the Jeep is trying to tell us.

This is a chrysler system so try the key on key off trick see if it works. What you do is turn the key on then off do this 3 times leaving the key on the 3rd time. If this works then the engine light will start to blink the codes to you. It will do it as such. 11 will be one blink the short pause then one blink, then longer pause and then the next code. Let me know if it works otherwise you will either need to have it scanned at a shop or purchase an OBD 1 and 2 scanner.

Ok here it is. Code 12 say's that the battery was disconnected. Code 21 is an O2 sensor problem we will have to look into that. Code 55 means end of codes. What you need to do is inspect all the wiring to the O2 sensor if all looks good then you will need to replace the O2 sensor. If by chance this don't fix it then we will need to get a digital multi meter and make a jumper wire or two and do some wire checking to determine if the PCM may be the problem (not very likely). After you replace the O2 then clear the codes. To clear the codes simply disconnect the battery for a min or so. Now if you have a factory alarm then you will need to do the relearn procedure as such, before reconnecting the battery put the key in the drivers door lock, now reconnect battery then cycle the door lock once, horn may sound three times to tell you it is activated. Thats it. Let me know how it goes.

The O2 sensor is in the exhaust system. It will be either in the exhaust pipe before the cat or in the exhaust manifold. Here is a pic so you know what you are looking for. Now what you check for is any bear or chaffed wiring or broke wires or? If all look normal with the wiring to it then replace it. It will probably cost about 60 buck.