2000 Jeep Cherokee wont idle

You could possibly have a bad EGR valve which would affect the vacuum which affects the idle. Check for a vacuum leak first then check the egr valve

Get the fuel mapping sensor looked at. what type of car is it?

sounds like blown headgasket

If it will crank but won't idle then it is the idle air control valve

Your only problem friend, is the temperature sending unit/sensor. Upon cold start, this sensor wil tell the computer to go into cold start mode putting a bit more fuel in and raising the idle a bit to keep it running when cold without using the throttle. Replace that sensor and your good to go. Makes sense, right?

In the PCM is where the Horn Relay is. It's location should be annotated on the underside of the cover you pulled off to gain access. One way to determine if it is the Relay or not is to find a similiar Relay (shape and size) and switch them. If this fixes the Horn problem it is the Relay and should be replaced.

check the throttle position sensor, sometimes it wont generate a code.

try pulling the plug out on the idle air control valve then start it if thats not it try the map then the idle control

battery and iac sensor cause this problem....... check battery conections and replace the idle air control valve on the intake....

Check your Idle Air Control (IAC) Valve; it may need to be cleaned.