the sending unit is nothing more than a float in the gas tank. Check in the trunk first and see if you can access the sender from there. you will have to pull the carpet up. If you can,you should be able to disconect the electical plug-in sending info the the gauge. try that. It can only be the sending unit or the gauge itself. I would start by pulling the power to each one.
The fuel guage is inside the gas tank so you MUST drop the tank; then it could be it's just sticking or it has broken off. No way to fix it without dropin the gas tank.
I would suspect the sending unit in the gas tank is at fault.
Try spraying them with WD-40 and let iot sit on there for a while. If that does not work try a 4 way lug wrench .
The diagnostic code 12 51 55 is actually not a code for the 1994 Jeep Grand Cherokee Laredo. The 12 means that it has started searching for codes. The 51 is oxygen sensor stuck lean. The 55 means it is finished searching for codes. Getting this means nothing is wrong.
if the guage reads full every time you turn on the switch, it's probably a short in the ground. you'll have to trace and inspect the ground wire from the guage to the tank. this will probably include dropping the tank. i would run the tank almost out of fuel 1st (makes it a LOT lighter).
You most likely tripped the NSS =Neutral Safety Switch. I'm guessing you saw a spark while you were messing with your lights. A mechanic can fix this for you.
Sounds like your cooling system termostate is stuck open.
Where is it stuck? on full or empty? if on full you got a bad ground, if empty you got a bad float.
fuel sending unit broken wire or gauge is bad
Most likely this is the result of a stuck thermostat.
Float could be stuck in the tank or the ground wire on the tank is broke.
a stuck rely. and some time spring in the button on steering wheel gets stuck too
The float that tells your gauge how much gas you have may be stuck or there is a problem with the system that sends that information to the gauge.
My 1999 Grand Cherokee just started this today. My first thought is that the fan relay is stuck in the "on" position.
Bad connection, blown fuse, or the float in the tank is stuck.
The fuel gauge could be getting improper signals from the fuel sensor. The sensor is located inside of the fuel tank.
A short time is normal, if it won't stop, your relay is stuck, replace it.
It can be stuck in 4 wheel drive or the idle control system needs attention.
Trouble code P1899 means: Park/ Neutral switch stuck in park or gear