vokera syntesi 29 combi-boiler gives different error codes. eg 01, 06, D1, ?

My boiler stop working giving error code like D2, D4, D1, 01, 06. Loose pressure lot. If I switch of from mains for awhile it works for a while then gives Code D4. If I switch off & on straight away from boiler it start working but if I leave it for longer it turns to code D2. Then I have to switch it off from the mains. Please help to find fault. Thanks