I looked it up, and CarMD seems to be an OBDII compliant reader. It won't be
the most convenient to use - you will have to look up in a book what the
codes mean rather than them displaying on the readout - but it should do the
job. The instructions will tell you how to "clear the codes" which will turn
off the light... at least for now. Write the codes down before you clear
them. They will be in the form of P----, like P0410.

There are three possible outcomes in this. First, the code may clear
successfully and never come back or only come back sporadically. Second, the
code may clear and come back in a few hours or days. Third, the code may
come back immediately. When and if the code returns the "check engine" light
will come on. If you post the codes we (especially the people here who know
the 850 well) can tell you what is probably happening and what you can do
about it.

Mike