The work is not usually the hard part. (With a few exceptions, such as timing belt/ water pump/ etc. which are multi step processes )
The troublesgooting/ diagnostics is what creates the reputation of a mechanic.
For instance, my 2004 Avalon with 160k, has a heating issue. The temp gauge would heat up and hover just over half (possibly low coolant?). Then at high speeds would drop to less than 1/4, still safe but weird. It would also blow warm/cool air when idiling, hot air during normal driving and cold air when the gauge dropped.
There are possibly two issues here but I don't know.
Flushed the system. Refilled coolant.
Replaced the thermostat (which was a pain on this car) based off internet diagnostics but no dice.
Now I need a qualified mechanic to properly diagnose the issue.