i charge flat rate also and on the few time with and problem that cant be found go back one time fee till it is found. then they pay for the repair. to just change parts till you find it isnt fair to the coustomer nor does it allow for learning.
you said he changed parts and then you think the gas valve was bad. but when you say it is still working means you arent sure it was the problem. probably was from what you have said but now the coustomer has paid for three calls and one control and one gas valve. how can this be fair to the coustomer? how is this fair to you also since you are the one being trained
changing parts till the problem doesnt happen any more is not service.
taking the time to find the problem and fixing it once is service. even if the repair has to wait till you can locate what is causing the problem
honesty,quality,and fairness is what we should strive for