I have the same 855, 5spd, non-turbo and fearing a repeat of my failed emissions test two years and some 32k miles ago, I changed sparkplugs, wireset, airfilter and cleaned the throttle body. Then I poured BG44k additive on a full tank and drove 300 miles over a period of 3 days to use up the gas in the tank. On inspection day, i got oil/oilfilter change and drove to the inspection center. Needless to say, the car passed easily with very impressive near zero emission numbers.

What I observed during tune-up was that the sparkplug tips were almost burnt away. Also the wires and the boots inside the spark well were aged and somewhat cracked. BTW, both were changed during the previous tune-up 32k miles ago when it failed test in 2002. So, unless you are getting specific fault codes requiring changing O2 sensors etc, I recommend the above tune-up. Total DIY project took a couple of hours, parts cost was about $100. Oilchange was about $60 (synthetic).

yt850,
1994 v855, 5spd, nonturbo at 103k
1984 MB300D turbodiesel at 174k