i had the same problem it was a vacum leak under battery box you will have to take the battery and battery box out this vacum line hooks to cruise control fix the leak and it will work fine the vacum line has a curve in it to find other vacum leaks let the engine run and spray carburetor cleaner on vacum line connection
When you think there is a possibility you might get stuck if you don't
My 1999 Grand Cherokee just started this today. My first thought is that the fan relay is stuck in the "on" position.
It can be stuck in 4 wheel drive or the idle control system needs attention.
Little or no heat from heater/defrost if t-stat is stuck open. Engine overheats if t-stat is stuck closed.
Could be,a stuck open thermostat,low on coolant,plugged heater core,faulty temp blend door actuator,
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.
Thermostat stuck open, if older car possibly clogged heater core- when run at oper temp the hoses going into/out of heater core be sure one is very hot - start with thermostat $$$ cheap & easier than a heater core problem!
If it's stuck in defrost your battery died and messed up the computer. Disconnect battery for 5 or more min. It worked on my 96
Stuck open thermostat. Plugged up heater core. Temp blend door malfunction.
Check behind the heat control panel to be sure that the vacuum hoses are all securely plugged into the control switch. As a safety measure on all cars, if you lose vacuum altogether, the default position for air direction is on defrost, so you can always see out of the windshield. Jake McCahn
Did you check the thermostat? If so, try the heater core. check the air mix door in the heater it may be stuck and letting outside air in
The air door under the Blazers dash could be stuck, or there could be a vacuum leak on the engine. The door opens and closes from vacuum, directing the air from heat to defrost.