how can i fix my boiler its a baxi megaflo 28?

i have a baxi megaflo 28 which will heat my hot water and downstairs radiators but when i select heating upstairs rads error E164 keeps showing on the display and the upstairs rad circuit will not heat. the main distributors for the boiler eventually got back to me and said this code indicates an airlock, though all my rads have been bled and the auto air vent in the hot press is working, i am at a loss to figure how the boiler can sense an airlock upstairs, it cant be in the boiler as everything else works and the only way the boiler distinguishes between and seperates the three circuits is via the three motorised valves in the hot press. has anybody else encountered this error, or know what it is ? please help