Drain boiler (again - you'll get good at that ) undo the 4 Allen screws (or 2) which hold the pump round part and pull it off. Don't damage the O ring.
You might find the plastic box on the pump drops to bits - new pump required.
There's a gap between the two discs of the rotating part (rotor) about 4mm. Make sure it's clear.
If you happen to have have a capacitance meter, measure the cap on the pump. You may find a date label on the cap too.
Power up the pump (could turn boiler gas off and turn boiler on) and see how easy it is to stop the pump with one finger. It should be quite difficult.
If it's easy, you need a new pump
Sorry if I was a little peremptory earlier!
You might find the plastic box on the pump drops to bits - new pump required.
There's a gap between the two discs of the rotating part (rotor) about 4mm. Make sure it's clear.
If you happen to have have a capacitance meter, measure the cap on the pump. You may find a date label on the cap too.
Power up the pump (could turn boiler gas off and turn boiler on) and see how easy it is to stop the pump with one finger. It should be quite difficult.
If it's easy, you need a new pump
Sorry if I was a little peremptory earlier!