What the hell mechanic is this? Thats bulls**t, if its a good mechanic he'd check it out no matter what.
What do you mean when ya cars not running entirely smooth? You done an oil / oil filter change, check spark plugs, battery, fuel injectors, ingnition system, timing / fan belts, all the regular things you need to look out for... Your ECU will be able to check to see if the sensors are working...