1997 volvo 850 monitors won't set..no check eng lite...driving for 3mos

Oh boy, these 97 850s are tough. I used to do them all the time but I had a scan tool that allowed me to manually run several of the tests and monitor the others. Volvo has a technical service bulletin that walks you through how to reset the monitors, I suggest you find that and see if it helps. Otherwise go to a shop that substantial Volvo experience and they should be able to help. Good Luck