Well, that is another issue: no (or limited) competence of the plumber.
At least here (Italy) we have two kind of workers:
1) generic plumbers (BTW also perfectly capable of installing a new A/C system or boiler, furnace, etc.)
2) specialized "boilermen" or "heat engineers", i.e. people that do the maintenance and repair of them
The level of competence of #1 above is - generally speaking - excellent, whilst the level of competence of #2 greatly varies, particularly when the issue at hand is not "normal maintenance" but rather a malfunctioning, and even more so if it is an electronic/controller related one.
Even if manufacturers do provide courses to (hopefully) keep the maintenance crew up to date with specific new machinery, these courses are often completely unuseful (as they are often theory only and very little real-life or actual field related) or they are terribly conceived or taught, with the result that only a very limited number of people taking them actually understand them fully.
To give you an example, I often deal with a "largish" plumber firm that has some 15 people, of which some 10 are "generic" plumbers and 5 are "specialized" on HVAC, A/C, boilers, etc.
Of these 5 people, only 2 (IMHO) are good in problem solving, and only 1 is at the level where all of them should be.
This latter guy is often called directly by this or that manufacturer to go and solve for them issues even 3-400 Km away, which should mean that he is a rare resource.
And every year the manufacturer engineers add some more complexity to the machines (no matter if for a real bettering or not) completely disregarding the need of forming the maintenance and repair personnel.
There is also an underlying education problem, some machines are getting so complex that only someone with a graduate engineer level has the competence to troubleshoot some issues, but of course no graduate engineer would ever dirty his/her hands in a plumber job.