Just thought I'd share this
Aircon was not working on my 2000 V70 classic - mate who's an aircon engineer topped it up but it hardly took any - still not working. In the meantime I bought a OBD tester/reader/eraser from eBay for my Jag XJ sport - plugged it into the Volvo to see if it would communicate, it did and came up with a code that corresponded to an aircon sensor pressure fault - reset the fault code and hey presto!!! working aircon and it's been ice cold for over a week.
Just in case you are chasing a gremlin in your aircon - check the OBDII faults,
Cheers,
Steve