how to reset a vokers boiler?

hi, I have a vokera minute 24m gas boiler, it's working ok but the pump is supposed to run periodically to ensure the heat exchanger does not clog up, but when the pump runs if fires up the gas as well and this is not supposed to happen.
Can anyone explain how to reset it to it's original settings so that only the pump runs when the boiler is not in use.
Thank a lot
jp