Sorry to bump this thread again, but the SES light is back on. Codes this time:

P0455 - Evaporative Emission Control System Leak Detected Gross Leak
P0135 - O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 1
P0161 - O2 Sensor Heater Circuit Malfunction Bank 2 Sensor 2

So the O2 sensor codes are the same as before. No misfire codes or codes about running lean this time, though. No change in power or gas mileage either (still averaging around 20). I cleared the codes and the O2 sensor codes came back within 50 miles, but not the EEC leak code. The car drives fine and has been without SES for months.

This time around I'm noticing a couple things that are indicative of the DISA valve---whistling noise part throttle, and occasional unexpected acceleration as if a turbo just kicked in---so I plan to go ahead and rebuild it before anything else. Before I spend money on the O2 sensors as well, is there anything else I should check, specific to the heater circuits? Maybe a fuse or relay has gone bad? Or maybe a connector is loose somewhere?