I remember once having some engine work done.. Idiot left the blang sensor (MAP or MAF forget which) not hooked up.. Got that fixed.. and several years later I'm going down the road and suddenly Limp mode. Said sensor is the very first thing you see on that car when you look at the engine, he'd left the vacuum line off.
Several years later the car went into limp mode. I pulled off freeway read the code, called the mechanic and he told me "that's in the emissiions control area" I said "Thanks" .. the other end of the same vacuum line.