Are these certificates difficult to get? I've considered getting stuff like that when I become an owner so I can DIY stuff because AC is a necessary luxury for me and I'd rather be able to repair/replace as quickly and cheaply as possible with no salesmen and middlemen invovled.
The basic small appliance grade, I the read the training material for a couple hours at night, paid $25 for the test, and passed with a 98% needing something like an 80%. You will need a "type II" to do modern home units. That is a proctored test, but Skill Cat will do you there from the comfort of your phone.
You will need something under $1000 of gear and supplies to get going, half that if you can rent an evacuation unit. (I can't find any near me.) You need to store your refrigerant outside where it won't get over 49°C, 120°F (assuming R410a) So it rates pretty high in the "pain in the ass" factor.
For modern home units, the circuit boards and motor controllers seem to be a common failure point and these are crazy expensive from the manufacturers anyway and hard for you to buy as an individual. If you have an older unit before variable speed compressors and fans then you can probably get commodity parts to replace them.
