Thought I'd post a follow up on this, as I resolved the problem back in July but have been too busy (or too idle :^) to reply. It turns out that the ECC was at fault. The first one I replaced came out of a pristine 850 in a junkyard...I wondered why it had been junked and now I know possibly why.......

I'm using a 12v LED and a pushbutton switch as shown in the pic. The reader seems to work fine. I get the 1-1-1 code for all modules except the cruise control (where I got a different, but normal code) and he ECC, where I get absolutely nothing. I'd taken a quick look inside the ECC and saw nothing ...

Update - I built the LED code reader suggested by jreed and it works great on all modules except the ECC (OBD-2 connector 6). When connected to connector 6 the LED comes on when I press the push button, but no codes are displayed. In contrast, when connected to other connectors (like 3 for example) ...

Thanks for all the helpful replies. FWIW, I replaced the blower motor with a new one several weeks ago (the old one was noisy) and all the fuses appear to be OK. My next steps are to build the ECC code reader that jreed suggested (great writeup - thanks!) and replace the ignition switch. Question fo...

I have a ’96 Volvo 850 GLT in which the ECC has completely stopped working and the blower fan operates on high speed anytime the ignition is on. Prior to this the unit was functioning well except for intermittent two orange blinking lights on startup. I’d had it diagnosed at the Volvo dealer and the...