Hitachi Yutaki S RWM-10.0NE not defrosting and ice build=up

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.

Could the problem be that the 4-Way Valve Assy is blocked?
Or which part makes the switch to heat the outdoor unit during defrost?
I have attached an image of the Valve during defrost cycle. Thank you.
Attachment 16309

Defrost is controlled by the sensors. You need to check their resistance against the resistance table.
You can test the reversing valve either with a magnet or by changing operating mode i.e. heating to cooling

Thank you for your reply.

Today I've decided to try and find the cause and after two days of learning how the 4 way valve works, I simply powered of the system and opened the case on the outdoor unit, then unscrewed and take out the coil controlling the 4 way valve and wile removing it, I heard the beautiful sound of valve switching and pressure discharge :)

Now it seems to work well and as a precaution, I ordered a new coil and valve.

BR,
Daniel M.