High Idle with Check Engine Light

1996 Outback Wagon with 2.2L engine. Car has recently suffered from an occasional high idle, above 1000 rpm and higher when the A/C compressor kicks on. Today it was idling normally and then started idling high and the check engine light came on.

Any ideas before I go and have the code read?

Also, the gas cap was not fully tightened when I checked it this morning.

Thanks, Matt