Thanks for the advice. I'm a novice at this. The car spontaneously went into diagnostic mode and stayed there. In case this happens again, I was hoping I could reset out of diagnostic mode myself without paying someone $100 to do it with a scanner. Our other car (Eldorado Touring Coupe) lets you enter diagnostic mode, read error codes, and get back out just by pressing some weird button combinations on the dash.
I did try google. That's how I found this site. Thanks again for your response.
DiverBob