What are codes 12 and 21 for 95 jeep grand cherokee 4wd

Check this:
Code 12 - Direct battery input to PCM was disconnected within the last 50 key-on cycles.

Code 21 - Neither rich nor lean condition detected from the oxygen sensor input or O2 (Oxygen) sensor input voltage maintained above the normal operating range.

Code 55 - End test.

Just need to check or replace the O2 sensor, is the only fail that your computer found there. Is need other details, dont dude in contact us.

Good luck.