Wonder if anyone can give me some advice.

I have a Ford Focus diesel. 1.8 TDCI

I have what I assume is an electrical problem. The car was left out in the sun for several days (maybe irrelevant and no, it wasn't in UK).

On starting up the car I noticed several problems that hadn't been there before. The main problem was the air con/heating etc panel was totally dead. Plus, the indicators work but not the indication on the dash board that they are working. The dashboard indicator of outside air temperature is just showing --. The symbol that looks like the engine warning light stays lit (not sure what it is as the manual is in Spanish). There may be other problems but I haven't spotted them yet.

I did run the dasboard test sequence but the only DTC code to come up was D900 which I believe is quite common

Can anyone suggest anything I can try to resolve this? I am happy using a spanner but am not a mechanic so nothing too complicated please. Could it be something simple like disconnecting the battery for a couple of minutes to reset things? If so, is there anything else I shoud do apart fro just disconnecting the negative lead?

There is a garage about 20 minutes away but not sure I would trust them with any more than basic servicing. The nearest Ford dealer is 30 kms away so would prefer to solve the problem myself if it is fairy straight forward.

Thanks in anticipation.