I bought a 2000 S40 with 169k the other day. The check engine light was on, but I figured at the low price of $1400 it was probably something manageable. Codes for air assisted control valve, turbo/super charger wastegate solenoid "A" low, evaporative emission system purge control valve circuit shorted, h02s heater control circuit bank 1 sensors 1 and 2 (to name a few). Plus the ABS, airbag, brake light, and flashing shift-up light intermittently appear in the instrument panel. There is no rhyme or pattern to these lights. Once the car got stuck in "winter mode" when the flashing shift-up light was active.

My question is: is it possible that the electrical system is faulty? I mean, I guess anything is possible, but the car seems to be running just fine, other than getting stuck in winter mode once. Has anyone heard of a faulty check engine system?

 

Try clearing the codes and CEL and see what happens.
Some may be old fault codes not properly cleared previously.