1998 DeVille: 1999 cadillac deville drver side heat blows hot pa...

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem. Deville 1996 1999 Factory Service Workshop repair manual

The passenger side air temp control has a problem. Check connections at switch. If you have a volt meter you can check the switch for continuity or ohms of resistance. No continuity, replace switch.

Could be low on freon, a problem with the main controller, or one of the blend doors.

There should be a nozzle on the left passenger side that has blue for cold and red for hot so if there is a light on the nozzle all you have to do is turn it to cold if you want it cold. Caddy's have dual heat so you can change what you feel coming out push the nozzle in and the light should go off. Therefore making the car on the same hot/cold.

your temperature mode door motor for left side is faulty, or your temp. controller on dash for left side is faulty.

Could be that the hvac control module has an intermittent problem or the air mix door control for the passenger side is sticking. Check for error codes (DTCs) using a code scanner, but may not show an intermittent mechanical issue. If you can get at the air mix control on the hvac box under the dash on the passenger side, you may be able to check its operation.

on my 99 devillle concours cold air blows from left and back vents but hot air blows from right vents

needs new bower motor

sounds like it might be low on coolant. Low coolant levels will stop the flow and circulation to the heater core, and also might be the cause of overheating

A/C systems are hard to diagnose without having access to the vehicle. One of the most common problems is undercharge (Not enough freon in the system) indicating a leak somewhere in the system. Other causes could be a burnt A/C compressor clutch fuse, defective compressor clutch, defective compressor/belt, defective High/Low pressure switches in the system, defective evaporator icing switch, defective wiring... Basically, you're better off getting the system diagnosed by a good A/C knowledgeable mechanic...