Took my mom's car back to the mechanic. They gave me directions to a spot in a neighboring community (about 15 minutes drive away) and told me to drive there, let the car idle for about 5 minutes, then return. Still no codes on the car's computer to tell them what the issue is. Plan is to take it back tomorrow for a full diagnostics check (about $150).

(Many years ago, I used to say "the only thing worse than having a car is not having one".)