Just turn it on and your dash will flash the codes. 1 flash pause 2 flashes is a code 12. Some system flash a 5 code reading. Disconnect battery for a couple minutes to clear codes. To retrive codes from others - turn key on not running then press off and warmer buttons on the climate control at the same time.
Press reset and recall at the same time or auto to exit diagnotics.
Other method to clear is pressing off and high buttons. Another method is similar pressing off and warmer to get codes then pressing high to get codes them press low after each message until clear codes appears . Press bi-level to exit.