2000 Cadillac Deville no heat on passenger side

More than likely, you have a failed actuator for the rh blend door, or it overtraveled and defaulted to full cold.
If you press info up, and on/off to enter diagnostics, you can access the codes for the ACM. Info button will scroll and the on/off will execute commands.

Air Mix Actuator Replacement - Passenger
Removal Procedure
Caution: When you are performing service on or near the SIR components or the SIR wiring, you must disable the SIR system. Refer to Disabling the SIR System. Failure to follow the correct procedure could cause air bag deployment, personal injury, or unnecessary SIR system repairs.

Remove the IP compartment. Refer to Instrument Panel Storage Compartment Replacement in Instrument Panel, Gauges and Console.
Remove the electrical connector from the passenger air mix actuator (2).
Remove the passenger air mix actuator (2) retaining fasteners (1).

Remove the passenger air mix actuator (1).
Installation Procedure

Install the passenger air mix actuator (1) to the HVAC module.

Notice: Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.

Install the passenger air mix actuator (2) retaining fasteners (1).
Tighten
Tighten the fasteners (1) to 1.4 N Â m (12 lb in).

Install the electrical connector to the passenger air mix actuator (2).
Install the IP storage compartment. Refer to Instrument Panel Storage Compartment Replacement in Instrument Panel, Gauges and Console.