1996 Jeep Cherokee Codes

If you had the battery disconnected, recently, that probably caused the code 12. It means the computer has lost battery voltage recently, for some reason. The code 55, means end of message.

The code 27 is an injector code. The code 43 has to do with ignition spark.

I'd go to the coil primary connector, take it loose, two wires B+ and ground. Use an led testlite between the two terminals on the connector, have a helper crank the engine, see if the lite pulses?

There is a possiblity that an o2 heater circuit might have shorted, causing a problem for the other systems. The injectors--coil--o2 heater circuits and whatever are on the same voltage circuit from the asd relay. I'm saying the relay is faulty, you need to do some testing.