94/95 jeep 4 lt. Seem to have error code 12 followed by 55. Manual we have doesn't show 12 as an error code - any clues, please?

Google those codes to find the problem.

If it cranks over but doesn't start, then it's not your starter. code 12 is a common code but code 55 is some type of sensor. You need to call autozone or advance auto and find out what the code is. It could also be an ecm fuse. Might want to check that. Electronics are touchy and tricky topic. Start with the basics. No immediate remedy over the computer/internet. Just me giving you you advice that may or may not work. How many miles on vehicle?

12 just means the system was reset recently 55 mean end of codes so don't worry about them 2 however code 51 is an oxygen sensor error (O2 Sensor) a bad or shorted O2 sensor can cause it to run very bad and get poor gas mileage.

12 just means the battery was disconnected within the last 50 start attempts from what I can find there are 2 types of codes for jeeps , system faults and input faults ,, the only 55, 56 I could locate were 55 = Right temperature door travel too small
56 = Right temperature door travel too large I have no idea what that means,, maybe someone whos better at jeeps could help you, sorry

Code 12 = Recently disconnected battery

Code 14 = Coolant sensor high resistance or shorted

Code 43 = Knock sensor or Electronic spark control circuit fault

Code 55 End read code.

Hope helps (remember rated this).

12 is usually a power disconnect, 14 is usually a map sensor 56 I'm not sure of...likely you mis-read it and it was a 55, indicating that the code sequence has ended (nothing code)...right after which, codes repeat.

yes it is a rich code 45 12 and 55 are nothing to worry about.probably a bad fuel pressure regulator

12 is a battery disconnect within the last 50 start cycles and 55 means the end of the diagnostic cycle.  Basically the computer found nothing wrong. The 12 code may seem incorrect if you haven't disconnected the battery recently, but from my experience the 12 code shows up almost all the time regardless.

12 - battery disconnect within last 50 key cycles. (nothing to worry about) 45 - Transmission pressure problem 55 - End of diagnostic cycle (no more codes to report)