We just had a '94 850 in our shop with the exact same code but it was a no start. It turned out that the main relay wires at that relay were in bad condition, not making contact. That's the large relay that sits in the panel just above the radiator on the left (drivers) side of the car.
Bad ECUs are highly rare SO before buying another, either inspect the wiring in that area (remove relay to be sure) or even try another relay as apparently that will cause a "no spark" and a false RPM sensor fault code.

With all due respect to independent mechanics...the only person touching your car should be a Volvo Master Technician with experience. Having said that, it's highly unlikely your ECM needs replacing. Take it to Volvo where they have the know how on troubleshooting.

You can obtain a salvage yard ecu for under $200. I did and it works as good as my oem unit. In fact, I sent the salvage yard ecu off to be modified by Upsolute and have had it installed ever since no problem and fantastic performance. My pefectly good oem ecu sits in a nice box in my closet as a spare.

The key for you is to determine whether or not the ecu needs replacement.

For about $175 you could pick up a salavage yard ecu and give it a try.

Hey sounds like a great opportunity to upgrade to one of IPD's performance computers! (If yours is a Turbo that is....). Checkout their website.


Mike

The cost the dealer will charge you to diagnose the problem will not be that high, and probably worth it.

The cost to replace an engine ECU could be high, but this is an uncommon failure. If they tell you this is the problem, then consider buying a salvaged unit vs. a new part.