My interior blower for the heat/AC/defrost isn't working anymore. All my normal lights and indicators are on or off accordingly.

It won't work under any circumstances; on "Auto", "Recirc", or manual fan.

I've seen where some have talked about a resistor being bad and others refer to just the blower motor.

Is there any way to troubleshoot this thing to know which one is bad?

Could a Volvo mechanic pull some codes from it if I don't see any flashing lights and there's nothing on the "on board message center"?

 

check fuses (and relays) first. Older models have a resistor - which governs all but the top speed (so if you were a Volvo 850 that worked on only top fan speed, its the ballast resistor). the newer models use a slightly different set up... Best way to check is to open up by removing the glovebox and lower dash panels on the passenger side and see if you are getting voltage to the blower motor. if you have voltage but no spinning - I'd suspect the motor.