I have ust serviced my 2001 volvo s40 and now the

hold in on trip button turn key on hold til light flashes or goes out

did the sevice light come on before you serviced it? when you "serviced" it I assume you changed the oil and now need the service light reset.
I bought the 1998 S70. it got time to change the oil. Being how I am I changed the oil myself. I had to call dealership about resetting the service light.
they said $29.96.
I said 29.95?!!! how much if YOU would have changed my oil and reset the light?
they said $29.95.
so I learned you need to access the OBDII to reset the light.after service. hope this helps.