how can i fix my item greenstar 30cdi ?

most likely the heat exchanger is clogging up or the water temp entering the property is alot colder due to the weather. if its the first then u may need to have the heating system serviced as it will get worse and replacing the part without having the system cleaned and treated will cause it to clog up again. also it could be the temp sensor but u will need an engineer to diagnose and eliminate each fault

kash
February 2013