Robert,

i dont have much experience with volvo check engine lights but i do have alot of experience with my 1995 maxima.i bought it used with about 75,000 miles on it.shortly afterwards,my check engine light came on and the dealer replaced the co2 exhaust probe.

they say that 90% of these check engine light problems are related to the co2 system.

it seems like around 100,ooo miles these co2 probes start to fail.unless you have a db2 code detector, you wont know for sure.

just about 6-12 months ago,the check engine light would come on after a highway drive at 60-70mph. then under some unknown condition, the light would go out.this continued over a 3-6 months period.then it finally just stayed on and i knew then that the other co2 probe on the other bank of cylinders finally failed.so i replaced the other co2 probe in the other exhaust manifold.

at least on the maxima,the co2 probe fails over time.i think they just wear out from all the heat and cycling they see and i am sure the soot doesnt help the situation.most of these are made by Bosch.i think the 850 is a straight (5) cylinder so you should only have one bank of cylinders and one exhaust manifold to look at.just disconnect the probe at both ends which is fairly easy to do then install the new one.the last trick is to reset the check engine light which on some cars means just disconnecting the battery to reset but on others you have to find the reset button.hope this helps.let me know.

rlk2525@juno.com