Well, many of us have sought out similar information, probably with mixed results.

2008 Volvo XC90 Owners Manual:
Go to section 02 Instruments and Control, then to paragraphs 54-55. As would be expected, the manual doesn't instruct the owner how to reset; i.e., can only be "reset by a trained and qualified Volvo service technician." I think most owners manuals will say the same thing.

That said, there are lots of sites on the web (You Tube, blogs etc.) that give advice on resetting Volvo service reminders, but many of them are specific to certain years and models only. (My daughter successfully found instructions for her 2010 S40).

ipd is usually a credible source; here's what it says: https://www.ipdusa.com/blogs/103/howto-reset-service-reminder-indicator-on-1998-2009-volvos

These ipd instructions work for our 2000 V70. But I cannot attest on whether it works for a 2008 Volvo model.

This is what I post every time this comes up.
I don't know if the second one goes "post P2".
I agree, the pushing and timing seem simple enough but each time I do it it takes me 10 tries.

Resetting the Service reminder indicator (SRI), model years -2001
The service reminder indicator remains lit for 120 seconds when the ignition is switched on.
Note! If the lamp flashes, it may be because one of the service parameters has not been programmed.
Proceed as follows to reset:
-Ignition position I
-Press and hold in the reset button for the trip odometer
-Ignition position II
-The service reminder indicator (SRI) starts flashing when the reset button for the trip odometer has been pressed for 10 seconds
-Release the reset button for the trip odometer within 5 seconds. The instrument gives an audible signal when resetting has succeeded.
Note! If the trip meter reset button is not released within 5 seconds the indicator is not reset. However the indicator will stop flashing.


Resetting the service reminder indicator (SRI), model years 2002-
The service reminder is displayed for 120 s each time the ignition is switched on.
-Ignition position I
-Press and hold in the reset button for the trip odometer
-Ignition position II.
Note! Turn the key to position II within 2 seconds.
-Hold the reset button for the trip meter pressed in until the original value has been reset.
Note! For all vehicles from the 2003 model year a general yellow lamp will light when the reset button must be released.
-Release the reset button for the trip odometer within 4 seconds. The instrument gives an audible signal when resetting has succeeded
-If the trip meter is already reset the reset button should be held in for a minimum of 10 seconds and a maximum of 14 seconds.
Note! If the reset button for the trip odometer is not released within 4 seconds, the service reminder indicator (SRI) is not reset.
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