It's been a couple of years since you had the car tuned up but it still seems to be running fine. Leave well enough alone, right?
On most vehicles today, malfunctions may be camouflaged by computers whose function is to keep the engine running as efficiently as possible as long as possible. When they no longer can compensate for mechanical discrepancies, they may just fail. The vehicle is disabled. Periodic diagnostic checks and maintenance can prevent this sometimes disastrous situation.
Symptoms of engine trouble on most vehicles today are subtle. It's not easy to tell if your car needs a tune-up because the symptoms are masked by computerized controls. Preventive diagnosis, done periodically, can keep your vehicle running trouble-free for years. Short of that, if you do detect a change in your vehicle's performance, make a note of it and report it to your technician for further diagnosis and corrective measures.