Why is the pump running constantly on a Greenstar CDi boiler?

The pumps on the Greenstar CDi range have a permanent live feed to them. If the pump is always running, there are generally 3 causes for this:

1. The speed control wire (purple, white and black) going to the pump is damaged - check continuity and connection.
2. The boiler has a weather compensating controller fitted (FW100 or Wave smart internet connected controller). The pump will run constantly to monitor the flow temperature and maintain heat curve.
3. The PCB is faulty and telling the pump to run when it should not be running.