The Ohmmeter

One failed (or failing) injector can cause the others on the electrical circuit to become overloaded and also fail. You or your mechanic can use a tool called an ohmmeter to ensure your injectors' electrical resistance is within spec -- any that fall considerably outside spec should be replaced.

Faulty Fuel Injectors

If your fuel injectors are on the fritz, you'll definitely notice a difference when driving. Left unchecked, faulty fuel injectors can damage an engine enough to keep the car from running, but there's usually time to intercept and fix the problem.

Faulty fuel injectors on late-model cars will make their presence known by causing a cylinder to misfire. This is because on newer fuel-injected systems, the injectors work in sequence, and when the engine misses a dose of fuel, it won't run smoothly and may suffer damage over time. It's less of a problem in older cars featuring simultaneous injection systems, because good fuel injectors can sometimes compensate for weaker injectors, allowing the engine to recover its sequence more quickly.

Still, faulty injectors can't be overlooked or ignored for long. Dirty or worn-out injectors can also cause preignition or detonation [source: Carley]. Detonation is a fairly common problem in which gas remaining at the end of the normal air/fuel burning cycle (which is set off by the spark plug) spontaneously combusts. Detonation is harmless in some cases, but the pressure has potential to break engine components and cause pitting and scuffing around the pistons. Preignition is when the air and fuel combust before the spark plug fires, causing severely increased engine temperature and damaged pistons. This problem is much more likely to pinpoint a leaky injector than detonation is, because it's a relatively rare occurrence in fuel-injected engines.

Leaky injectors can also cause the car to flood -- that's when the engine is shut off with excess fuel trapped in the system. The fuel then vaporizes, settles and purges when the car is restarted, which can cause extensive damage [source: Allen].

Try cleaning your injectors before considering a replacement [source: Carley]. When replacement is the only option, take heed: At the very least, you'll need to replace only the faulty injectors [source: Carley]. They can cost several hundred dollars apiece, depending on your car, but you can ask your mechanic to look for rebuilt or remanufactured OEM (original equipment manufacturer, or factory quality) injectors to pare down the cost.

Whether your injectors are new or old, now's the time to start a preventive maintenance regimen. Read on to the next page for lots more about fuel systems and taking care of your car.