Been tracking down some codes that pop up occasionally on my '96 850. Was getting misfires but it looks like new plugs, wires, dist cap, rotor and a throttle body cleaning have taken care of those. The only one I still see is the P0172 (System too rich). Looking through the archives it seems to point to the O2 sensor. The car has 47K mi and has still has the original sensors. Most of the 47K mi is local so I'm guessing this might be a bit harder on the sensor. Any comments would be appreciated.

Tnx,
Dave

Check your vacuum hoses.. then can cause false 02 sensor codes 435 and 436.
I have the same on mine but I'll wait till spring when its warmer to looks at it. I hope by then its not too late because running rich will clog up the o2 sensor.

Doug,

Tnx for the comment. Bottom line is that I'm not seeing any of the O2 sensor codes, just P0172, which is system too rich. I did find some non-volvo information which seemed to indicate that one of the possible causes is dirty fuel injectors. Apparently this code shows up if the engine exhaust is richer than the computer thinks it should be based on the injector timing. If the the injectors stick or don't close cleanly this can happen. After reading this I put some Techron injector cleaner in with the last tank of gas. Need a few days to see if this helps.

Dave