My 1996 850R starting having problems and the check engine light came on. Dealership said it was a misfire code and tried to find the problem for 2-days. They called me today and said they put the wrong plugs in at my last service. My question is what type of damage if any did this cause. The car would not
run smoothly and would almost stall when stopped. These problems only occurred after 800-1000 miles after plug instillation

thanks!

That's somewhat strange, mainly that a dealer service writer would tell you that they installed the wrong plugs but there's only 2 types of OEM plugs for 850s and IMHO, putting the wrong plugs in your engine shouldn't normally do that.
At any rate, the worst thing that's probably occured might be "gassed oil" so ask them to change your oil again if they havent already done so.

I recently changed the plugs on my 850R. The correct Volvo ones are the single electrode. The ones I took out (put in by my local dealer)were the three prong type (not recommended for the R). The only effect I noticed with the "wrong" plugs was a very slight, about 5%, decrease in gas mileage. It did not seem to affect the car in any other way.
John Scales. (1996 850R)

Glad we're on the sparkplug subject. I have 4 electrode plugs in my 1996 854 GLT (non-turbo). They were in it when I got it and appear pretty new...they say Volvo on them. What types of plugs are best? I've heard that Bosch Platinum single electroded are good...but doesn't the manual call for a common plug like Champion or something?

Glad we're on the sparkplug subject. I have 4 electrode plugs in my 1996 854 GLT (non-turbo). They were in it when I got it and appear pretty new...they say Volvo on them. What types of plugs are best? I've heard that Bosch Platinum single electroded are good...but doesn't the manual call for a common plug like Champion or something?