Well, I performed additional in-depth wiring testing according to the Volvo manual. Voltage measurements indicated that "diagnostic signal from power stage" of 1V means no signal. So I felt a little more confident that the power stage was the cause of the problem. I had a hi-power FET sitt...

I have checked the wiring between ECC and power stage and there is continuity. Is there a way to bypass the ECC controller and make the power stage turn on? I guess we would need the schematics for that... BTW I found a blower power stage online for $20 (it's a replacement, not Volvo OEM). At a junk...

My main question here is: does code 4-1-8 necessarily imply a bad power stage?

After all it says "no control signal to blower fan power stage". Could that be caused by some sensor somewhere? I tried to locate the outside temperature sensor but I didn't find it and it seems to work anyway.

I checked the MOSFET with a DMMs (Fluke 77). It seems like it had enough voltage to turn the device on (source to gate) and then there was conductivity between source and drain. When putting my finger across the pins the MOSFET would turn off and then I had an open circuit between source and drain. ...

I have similar issues with my '95 Volvo 850 turbo w/ ECC. I pulled the codes: 4-1-4 Driver's side interior temp sensor inlet fan seized. 4-1-7 Passenger's side as 4-1-4. 4-1-8 No control signal to blower fan power stage. I fixed the driver's side temp. sensor. I tried to fix the passenger's one but ...