My S80 2.4 is 2000 reg. The internal blower fan runs full speed and will not switch off.

All online discussions have pointed to the resistor module but .I have replaced the resistor module twice with new ones and have tried swapping the heater control module with a second hand ones - twice. I'm at a loos at where the issue might be. There is 12V permanently on the main Red and Black wires. I cannot detect any square wave signal on the blue and yellow wires. The blue and yellow wires both register 0.3V against ground (black) when the ignition is on.

It could be that the new modules (from China via ebay) are also faulty or burn out immediately. But that seems unlikely. I expect that even new resistor modules supplied by Volvo come from the same Chinese factory.

Any ideas?

 

Yes,

"It could be that the new modules (from China via ebay) are also faulty"

Perhaps choose a different brand for your replacement part