clean all your contactors and make sure none are sticking, they should have good contacts and be clean.
and yes you are correct, the access routine is horn button + up function while turning on the key, but this only gains you access to the passcode menu, you need to know the correct passcode to gain access to the main menus. Raymond was notorious for changing these randomly to new codes every month. They NEVER keep them at the factory default settings which are Ray1 and Ray2. So unless you know the current passcodes you will not gain access to the service menu's.