Hello, you can save an hour on this by telling me what the code read out for F1 04 is.... these codes are helpful indeed, about 30% of the time, the other 70% of the time they can be misleading or in more than a few cases, entirely bogus.
Regarding the temperature loss in the supply air ducts from 34C to 29C... the duct is either not as well insulated as it should be, OR... its leaking badly...OR its very long, and it is very cold outside and the velocity is so low that the air is cooling off.
There is another factor, thermometers are all 1 or 2 degrees C off either way, high or low, if you are using two different thermometers, one could be reading high, the other low, and that would give you a bogus result.
When measuring the air at a register the thermometer must be placed inside the register, not outside where the room air mixes with the supply air and gives a false read.
Let me know, we can go from there.