potterton ultra 2 switching to lockout after draining and refilling system?

after draining the system to fit new rad valves the system was refilled and turned back on it fired up ok then it switched to lockout. the heating engineer made some enquiries with a collegue and was told that it is the pump that has gone i found that strange as it was working on the morning of fitting do you have any idea if this is right or will it be something else? i would appreciate any help thank you