I have an Ariston EuroCombi boiler (model A/23 MFFI) which has become temperimental in refusing to supply any hot water.Sometimes it works but most of the time now doesn't.First noticed when tried to run the heating about three days ago and no hot water either.Bled all rads and pump and two bleed screws situated under where boiler is mounted - no avail!.At switch on it doesn't ignite the pump starts (it sounds like),fan starts for approx 7 seconds then all cuts off leaving little red LED on.Followed fault finder in manual to where it says check air pressure switch.Removed air pressure switch noting what went where and tried gently sucking and blowing through tubes and hearing clicking noise (obviously i do not know how to test this ! is there is a way or do i just replace ??) Put back switch and put back together and left for the night.Tried at lunchtime and hey presto ! hot water and heating - till the thermostat switched the burner off and back to square one - tiny red warning light.I think i've mentioned everything. Can anybody advise ?? would be greatly appreciated.Thanks