When I was throwing codes repeatedly due to a persistent ECU problem, I invested in a ScanGauge II. I had seen it on IPD's web site at this URL:

http://www.ipdusa.com/Volvo-S70V70-1998-2000/Tools/Scan-Gauge-II/p-128-730-2536/

However, I bought it directly from ScanGauge at this URL:

http://www.scangauge.com/products/

This device will not read ECC codes, nor will it give you the code definition. To get that, I enter the code here: http://www.aboutautomobile.com/DTC

It will clear your codes and reset your CEL; it will NOT reset your "SERVICE" light.

Klaus is quite right about rarely throwing codes and using a scanner at the parts store. Mine rarely comes out of the drawer, which, I guess, is a good thing. I told all my friends that I would be glad to read their codes for them, if they got a CEL.