If you get the orange blinking lights of death at engine start (the recirc and AC buttons have LED's in them), there will be one or more error codes stored in the ECC. You can make a code 'reader' with some wires and a 12V light bulb (LED is better):
https://www.matthewsvolvosite.com/forums/viewtopic.php?t=79406

I used large paper clips, straightened, to connect the 3 wires to the OBD box. The OBD is under the coin tray in front of the shifter.

If there are codes, write them all down, and then clear them. Unplug the 'reader' and restart the engine, then shut off the engine and check the codes again - these are the ones that you need to deal with.

The vent motors normally are very good and last forever. The plastic used to turn the vents is another story, they tend to crack and fail. The motors are all held in place with T15 screws, 3 of them. Only one, the ventilation damper motor on the driver's side gave me a problem - that one opens/closes the dash vents - and I had to remove the upper dash from the car to get at it.
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Keeping it running is better than buying new