Hello all, new to the forum. I just purchased a 93 850 sedan for my son and the heater will not always come on. Sometimes restarting the car will make it work however it will not cycle thru the blower positions IE floor dash defrost auto. Hot air blows from the vents but not much. I've done some research online and understand it may be the resistor pack but it may also be the control module. Sometimes the recirc and compressor lights will flash at start up other times it will not. I'm mechanically inclined somewhat but am new to Volvos. Any help would be greatly appreciated. Thanks!

 

When they flash I would pull the codes.
This site will tell you how to pull the codes.
http://www.troublecodes.net/Volvo/
This post will tell you the positions of the OBDI plug.
https://volvoforums.com/forum/volvo-850-16/list-diagnostic-trouble-codes-dtc-9856/