OK, this is defintely a new one on me. The check engine light's on in our non-turbo 1995 850. My trusty OBD-II reader says no codes but displays all the sensor data, etc., normally and claims to be communicating with the car both as a Volvo and as generic OBD-II.
Using the diagnostic connector blink code procedure yields 1-1-1 (no fault detected), confirming what my OBD-II reader says.
Instrument cluster?
JRE