CEL is ON - Multible Error Codes - Where to Start??

[feathersjr]

Picked up another Legacy wagon LS, 91, with only 76 kmiles. That little old lady story. Anyway, the CEL is on and a local shop said the codes were for the following: Knock Sensor, Purge Soleniod, Temp Sensor-coolant.
I plan to dig into it this weekend and am looking for some direction.
I've got a manual for the EGI System, but most of the procedures involve the Select Monitor, which is on my list of tools to steal
I think it would be unlikely that all of these things have failed at the same time. I drove the car home and it seemed to have full power, it also just passed CA smog inspection.
Any feed back??
TIA
feathersjr

[Brat4by4]

Reset all codes. See which ones reappear. Then start replacing parts.