A few items: 1) CEL light will come on after proper cycle if problem is still there. That is why it comes back after 20 minutes. With this in mind can you check immediately for "pending" codes on your scanner? This may tell you if you have an immediate fault that is going to result in CEL light. How quick it turns on is a clue that a part is bad to start with. In the same vane, can you check your fuel trims - this will shed some light on whether you have an air leak. Smoke test is your friend.
2) Generic scanners are not always the best - sometimes they display wrong codes. Invest in VIDA or find a shop with a high end scanner that does all Volvo scans. This could provide more insight.
After that as mrbrian200 says - look closely at ETA
Help, Advice, Owners' Discussion and DIY Tutorials on Volvo XC90s. The XC90 proved to be very popular, and very good for Volvo's sales numbers, since its introduction in model year 2003 (North America).