From MACS technical think tank
Many manufacturers are using some form of control module to regulate blower speed. This allows infinite speed control instead of just a few choices. Recently, we heard about a 2005 Saab 9-3, with a “no blower” complaint. The problem was traced to a failed control Saab calls a “ventilation fan control”. Once finished with the install, the technician turned the key on to verify the fix. No blower! But the tech knew he diagnosed correctly. On a whim, he adjusted the fan speed, and as soon as he did, the blower came on and worked normally. Cycling the key off/on though caused the blower to stop again, until the speed was adjusted again.
Since the system is all computerized, his first thought was the control modules were seeing something they didn’t like. Might you think the same thing? But he did a little homework and found out the blower was working just as it was supposed to. The blower will not come on unless the blower speed is altered or the car is started.
If it’s a system you’ve never seen before, do your homework! Know how it works so you don’t assume it’s broken when its not.