Hi guys.
Can you please help me with my Heat Pump system Hitachi Yutaki S split ?
RWM-10.0NE indoor
RAS-10WHNPE outdoor

It worked well until 5 days ago. First thing I noticed is that the ice was building up on the radiator of the outdoor unit.
I also realized that, when it has to stop heating cycle and switch to defrost for outdoor unit, there is no longer the sound I normally hear: a high pressure gas discharge sound (also when switching back it was making a sound as pressure building back up in the inside copper pipes).
Now I no longer hear those sounds. Only that the fans stop and the compressor is running but the ice is not melting. Also inside the outdoor unit case, ice was building up but I melt it with warm water.
I think the 3 way valve that is supposed to switch the heat exchange from indoor circuit to outdoor one, is blocked but I have no idea how to test it (I learned it is switched by a 230v double pulse ON and double pulse OFF signal to change valve position).

I am not an engineer like you but no one here (including Hitachi "service") is offering help.
Please guide me on how to find the problem. I have beginner electronic knowledge.

Thank you.