Antonio,
There is limited problem solving you can do on a Z4000 by yourself. some items you can check are:
check both throttle pot for security of mounting, wiring, the pot itself should be 0 ohms in OFF position and is linear up to 10K ohms at full range.
check the 12A circuit breakers.
check the power switch.
Verify that the fan leads are plugged into the power board. The fan is polarity sensitive.
Other than that, I would send it to GGG or MTH itself.
good luck,
bruce