1998 Cadillac Catera Out of patience and money/heater

It is a dual zone system, I don't see where it defaults to full cold in the event of a failure, and I can't be sure that both actuators have failed. You have a few things to be concerned with. Is the lines to the heater core hot, indicating that there is hot water flowing through it. There is a hot water control valve that controls this, there is also an auxiliary heater water pump, description as follows;
Auxiliary Water Pump Description
Two electric engine coolant pumps are used in conjunction with the belt driven mechanical pump.

The continuously running electric pump mounted on the RH strut tower assists the flow of warm coolant to the heater core at idle when the ambient temperatures are low. A failure in this pump would result in poor heating performance, especially at idle.

The second electrical pump is mounted on the LH of the radiator, which provides the additional flow of coolant through the radiator. This pump is energized when the coolant temperatures reach 90 °C (194 °F)

I would say if you don't have hot water to the core, the problem is not actuators. Check the vacuum line to the hot water valve, I don't find anything that says when it has vacuum, just when it blocks flow. With that it is either going to have a vacuum signal until max AC is selected, or it is not going to have a vacuum signal until max AC is selected. Here is it's description;

"Â The Water Valve. The hot water valve shuts the entry of the heated coolant to the heater core when both the driver and the passenger temperature settings are requesting a maximum A/C. Thus, the cooling efficiency is increased.

A scan tool would be required to check for HVAC codes, as well as operating the actuators to see if they are moving. The dash does not need to come out for actuator replacement.

If you have low heat from both sides, it sounds more like a cooling system problem. One side could be slightly warmer just due to proximity to heater core and hvac case design. Check that the vehicle is getting up to operating temp, and that the heater valve and auxiliary water pump are working.

I can't say that you don't have actuator issues, the mechanical portion is something you may be able to rule out. Think of it this way, anytime your operating in 'auto' the A/C compressor is operating. If you have no hot water flowing through the heater core, you are basically operating the same as though you selected 60 deg on the panel. And the same rule applies, the closer outlet to the core is going to be colder. The physical aspect is something that you may be able to check yourself, like whether or not hot water is passing the valve. On a catera, it is not real easy to access this valve, or the inlet/outlet hoses to the core, but it can be done. To assume it is actuator problems is just that, an assumption. No manual will provide you a method for checking the actuators, It will require a scan toll that can read data, and possibly perform functions like actuator control.

The valve is located at rear of engine, behind the intake, black/three hoses and vacuum line. I can tell you this, the manual is not going to give you a diagram of the doors. Here is the chart for improper delivery and temp, from GM service info.

Service Information
1998 Cadillac Catera | Catera (VIN V) Service Manual | HVAC | HVAC Systems - Automatic | Diagnostic Information and Procedures | Document ID: 283631

Air Delivery and Temperature Control Check
Improper Air Delivery Step
Action
Value(s)
Yes
No

1
Was the Air Delivery System Check performed?
--
Go to Step 2
Go to Air Delivery System Check

2
"Â Install the scan tool.

"Â Select the actuator tests from the Air Conditioning SPECIAL FUNCTIONS menu.

"Â Select one actuator test at a time and command the actuator from 0% to 100%.

"Â Monitor the air flow when commanding the selected actuator (e.G, good air flow should be felt when the floor actuator is commanded to 100% and none when commanded to 0%.)

Does the scan tool display the status ACTIVE when the actuator is commanded?
--
Go to Step 3
Go to Step 4

3
Is good air flow felt when the actuator selected is commanded from 100% to 0%?
--
System OK
Go to Step 4

4
"Â Check the actuator that failed for mechanical binding. Repair as needed.

"Â Check the circuit wiring between the heater and A/C control and the actuator. Repair as needed. Refer to the applicable DTC table related to the actuator for assistance.

Actuator replacement procedures would be shown, from that you could remove them and exercise the doors, but that would not condemn a faulty actuator. Try disconnecting both battery cables and shorting them together. What this does is clear memory and faults. My thinking is, if this is a default in response to a fault, it may correct itself. Mind you if it is a hard fault, it won't take long then for the system to default again. Or, try this recalibration procedure.

Re-Calibrating Actuators After Power Loss
After disconnecting the battery or experiencing a complete power loss, synchronize the electronically operated actuators as follows:

"Â Turn ignition to the RUN position.
"Â Simultaneously press the AUTO and OFF buttons (located on the heat and A/C control head) for at least five seconds.
"Â After initiation, the stepper motors will cycle from one stop to the other. You may hear the motors cycling while calibrating.