Holiday rambler aluma lite 454 tbi too rich won't start

Code #12 is a good code to have. The IAC is removed with 2 bolts once removed look inside around the pintle sometimes carbon will build up causing the pintle to not fully close. If you turn the key on holding the IAC you should see about 3/8 in movement. Carbon can build up in the pintle seat also. It can be cleaned out with carb cleaner and Q Tips. What RPM does it show at idle when it does run?