ok i just tried it.
and I got into the diagnostic mode.
here is what I found.
when you turn the temp knob all the way to the coldest, it seems to be in "light + LCD test mode" as all the lights on the aircon unit light up and all the icons on the LCD are visible too.
next if you turn the temp up slowly, I think 2 or 3 degrees up, you will get in to what I think is the sensor test as I received a 20 meaning no problems, you might receive a different number here NJR 34GTT since your aircon is playing up.
the next few modes I have no idea what they are, they gave me a 5 and a 41 from memory.