worcester 24irsf boiler not turning off when ch on at 2bar?

hi my worcester 24i rsf combi boiler is approx 6yrs old and recently when on ch demand it is failling to turn on and off allthough it did do once after beeing on for 20 mins at 1.5bar after it fired back up it did not turn off again untill i turned it off after 50mins at this point it had reached 2bar can enyone help please