Have also caught a dodgy air conditioner mechanic trying to “fix” a problem by applying the placebo effect.
Our office was hot and stuffy all the time. Middle of Winter, -3deg outside - hot and stuffy. Hot Summer day - it’s hot and stuffy. Many complaints were made to the building management, multiple visits by air conditioner mechanics, who hung anemometers from air vents and pronounced the system to be working correctly. It was still hot and stuffy.
One day, with much fanfare, an adjustable thermostat was installed in a prominent location, to fix the problem once and for all. We were warned that it would only adjust the temperature +/- 3 degrees and that any change would take some time to be felt.
It was still hit and stuffy. We turned the control as far down as it would go... a day later, still hot and stuffy. Turned it all the way up. Didn’t get any hotter.
Our boss said: “this *$&*ing thing isn’t connected”. He grabbed the cable to the thermostat and started gently pulling on it. There no resistance and soon he had 10 metres of cable piled at his feet. The end was cut off cleanly, proving that it was a dummy control that had never been wired up.
The actual problem, diagnosed and rectified by our junior tech, was that every air outlet had a butterfly valve in the back of it and every one was turned off. I’ve never trusted air conditioner mechanics since!