can't find the bulb!

Fast blink usually occurs when a bulb has failed, but might possibly happen if one or more bulbs on the side in question have gone high impedance (bad contact) and are taking less current than usual.

Switch on the hazard lights on a dark night, and see whether the two sides flash with equal intensity. Or try cleaning the contacts on all of the bulbs on the 'fast' side - and make sure that any earth wires make proper contact with the bodywork (although a poor earth would also affect other bulbs - tail/stop etc. - in the same cluster).

[As far as I'm aware, the bulb failure unit doesn't monitor the operation of the direction indicators.]

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Don't some Volvos have fast-blink or bulb-failure indications when there are mismatched bulbs, like when one has been replaced with a non-exact replacement?

Might be worth checking to verify that the bulbs match.

Gary

The bulb failure sensor is a bit too sensitive and will turn on the indicator with mismatched bulbs. The fast blink is separate, it's done by the flasher itself. Old ones were electromechanical and blinked faster with less load because there was less current to heat the bimetal strip so the cycle was faster. Modern (as in the last 30 years or so) flashers are electronic and may be excessively sensitive.