These guys are exactly right! STOP trying to troubleshoot problems! This is how HI get their behinds in a vice, overstating what the cause is. If something does not work properly and using normal operating controls, you have identified an issue that requires the services of another profession.
In some cases and areas of the country you can get hung out to dry for going beyond what your license allows and step over into another profession’s area of license. If it is working normally, move on, if it not, identify the problem, then move on. We need to keep hammering this home until everyone gets it. We are inspectors, not troubleshooters or servicemen. This is why so much of the public has misconceptions about what a home inspection is and what it entails. I am trained and certified in HVAC but I do not even carry a set of gauges on my truck. If it does not work using the normal controls at the thermostat, I call for service by a licensed HVAC contractor. If the customer asks me what is wrong, I tell them it is not responding to normal operating controls and that it could be a myriad of reasons why it isn’t working and that they will need the services of a licensed HVAC contractor. NOT their cousin Willard who is “in construction” or their nephew who is a duct monkey working a summer job for a local HVAC dealer.
In this case, call a plumber, slow drain, not draining properly, take your pick.