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Called out to this boiler 475-75-18 today with 60 light flashing on demand for CH or HW. Fan not runnng. Pump proving fault i understand. Pump runs, no air in boiler. The CH differential valve pin is not lifting on demand. I can force the boiler to light by holding up the microswitch. I changed the CH diaphragm, cleaned thepin and greased. Also I made sure the small sensing pipe wasclear. The div valve is relatively clean (no gunge). Still the 60 degreeflashing. I had a spare pump head so changed that just in case. 60 degree neonstill flashing..... I removed large DHW press diff valve to access the front section of the CH valve behind. I then removed the front CH diff pin, cleaned, lubed and replaced. Still no joy. I tried to get the bypass pipe off to check for blockages but it wouldn't budge and I didn't have the O rings, so as time was getting on I went no further .... today. Am I right to assume the entire div valve needs stripping or replacing to rectify the problem? Anyone else had this problem?