Have a look at this, I found it on internet and haven't tried it my self but it seems creadable.
Reset Volvo 850 XC70 S70 V70 C70 S80 Service Light
There are two diagnostic units(it says A and B on the cover) located adjacent to washer reservoir under the hood.
1. Turn on the ignition and open the module A and insert the flylead into socket 7.
2. Press the button on the diagnostic unit 4 times, briefly but firmly.
Then the LED illuminates and stays illuminated, and the system is ready to accept manual code input. A code number for resetting the service reminder light is 1-5-1.
3. To do this, press the test button once and release it. And LED illuminates again.
4. Then press 5 times and wait for the LED to illuminate.
5. Press the button once and LED should flashes several times in succession and the reminder should have been reset.
6. Switch off the ignition and replace the flylead in its holder and close the cover.
850 (1996-1997) and S70 and V70 (1
Wednesday, February 21st, 2007 AT 1:55 PM