The factory service manual's description of diagnostic mode is pretty bad, requiring you to jump 10 pages back to check a referenced diagram, as well as incomplete descriptions of what actually happens. I think it's pretty incredible that someone at Ferrari spent time creating a troubleshooting feature that purely benefits the vehicle owner rather than the dealership service network. Of course, there is also an official Ferrari HVAC diagnostic computer that can be plugged in to perform these same functions. I dove into this topic when I started getting an intermittent E8 code for the electric heater water pump. Every time I test the pump it works fine, and it usually works fine while driving, but occasionally on the first start of the day I'll get the code. Switching the system off and back on again clears it and the heater will work for the rest of the trip. I took the factory service manual information as well as my own experience and combined them into what I think is an easier-to-use PDF with an accompanying video. Ferrari 348 Climate Control Diagnostics Mode Instructions PDF (feel free to distribute): https://tinyurl.com/yxlmdhwc How to Use Diagnostics Mode - Video: Hope this helps people get their system working perfectly again!