On Volvos made for the last 5-6 years the light will go off on its own after the problem ceases to occur and then the car is driven for a further 2-4 "trips". A trip is running the car through a varied driving cycle that is probably more than 20 miles for most people's driving styles. This allows all the self-diagnostic activity time to complete.

This is all part of the EPA Clean Air Act requirements part of which is this stuff that sometimes get batted around in here called OBD-II.

The person you spoke with wasn't wrong totally. His experience might be that many customers get impatient before their driving routine would allow the system to extinguish the light so coming in and getting it read out and turned off isn't a wrong answer.

Virtually all cars since 96 lost the capability to get codes w/o some type of scanner tool due to the OBD-II requirements. OBD-II requires certain info be displayed and certain things be stored when a fault occurs. This caused changes to the computers that made them lose the capabilty to do maunal code display. In effect the computers jumped out of the punched card era and into the serial data transmission era like other computers.