I have an '05 CTS and the Service A/C System code came up on my screen. The pressure was low and my local repairman (not a dealer) serviced my air conditioning and said it's fine. HOWEVER, he could not clear the message and it still comes up everytime I start the car. MUST I take this to a dealer to clear this code? I tried the f1 and f6 trick, did not work. Can anyone help?