Vaillant ECOmax 824E constantly runs with "06 Boiler shut down"?

i recently had a leak in the 6" rubber pipe in my vaillant boiler. I drained the system and replaced the pipe. i pressurised the system as in the instalation manual but the boiler mainly runs on status code 6 which is Boiler shut down on CH or DHW thermostat. when i run the hot tap the boiler fires up and runs normally on the hot water side. with tap off it goes back to status 6. when heating is switched on there is no change from status 6. any advice would be appreciated.