This is also why I own 2 shitbox cars.
I’m a good amateur mechanic. But I cant guarantee I wont order the wrong part, be sent the wrong part, have misdiagnosed the problem, the job turns out to be a nightmare etc… So as long as the other one is running, I have time and ability to sort it out.
Thats the thing. If a customer forgot to mask off an alarm panel or two not taking the 5 minutes to do it just makes you look like a fucking asshole. If they didnt mask off a bunch of stuff then you call them, tell them that either you need to add to the estimate or rebook the job.