Good morning my friends

My 2008 Lennox XP14-036 heat pump never worked well...
The first year I kept calling my contractor...
The compressor failed after 9 months of use...
I found myself,finally, that the lowpressure switch was defective (it was turning off constantly the compressor
They replaced the compressor,the low pressure switch as well as the 2 expansion valves and dryer....

Now as the automn arrived I start using the heat pump (I did not use it in the summer,afraid that it broke again!!!
It worked not bad for few days and now it just blow cold air.....
I checked the indoor filter and blower..OK as well as the outside blower..
Could it be that the system loose is R410 fluid
How can I test for a R410A leak
At these so high working pressure are there special requirements when welding the gaz circuitry
Thanks in advance for your help and advices