If your car’s “Check Engine” light is glaring at you, it’s probably because the oxygen sensor is malfunctioning. That’s right, the oxygen sensor. It’s a little device that’s a mystery for most drivers but its misbehavior is the problem that most commonly triggers a Check Engine light. The oxygen sensor unseats the formerly most common Check Engine light culprit: a loose gas cap.

Even though a car seems to be behaving normally, a faulty oxygen sensor will cause the engine to start gulping down gas. This problem can cause up to a 40 percent reduction in fuel economy.

The oxygen sensor, developed in the early 1980s, is an essential part of the car’s emissions control system, says John Nielsen, director of engineering and repair for the American Automobile Association (AAA). The sensor is about the size and shape of a spark plug and protrudes into the car engine’s exhaust stream. It determines if there is a lot or a little oxygen in the exhaust, so the engine can make adjustments to the amount of fuel being used in the engine to run at maximum efficiency.

Oxygen sensors in older cars fail for a variety of reasons. In some cases, sensors are fouled by gasoline additives or oil from worn engines. Newer oxygen sensors can last 100,000 miles if conditions are right, but often problems occur sooner.

Modern cars have two to four oxygen sensors. The sensors simply screw into place, but reaching them can be a problem for do-it-yourselfers. Additionally, since the exhaust subjects the sensor to extreme heat, it can “seize” (become frozen in place) and be tough to unscrew. A new sensor comes with anti-seize compound to apply to the threads, but the compound should never be put on the sensor itself.

As a first step, a car owner can look under the hood to see if there are any wires or hoses disconnected. In some cases, a wire leading to the oxygen sensor could be broken or burned out. If nothing obvious is visibly awry, it’s time to go to a trusted mechanic. Reputable garages use an expensive diagnostic machine called a scan tool — not to be confused with an inexpensive code reader — that can watch the operation of the engine in real time and see if the oxygen sensor is actually the problem.

“Most motorists would be well served to find a shop that they trust and take their car there for all oil changes and tire rotations,” Nielsen suggests. “Then, when they have a problem with something like an oxygen sensor, they trust what the mechanic is saying rather than thinking that they’re trying to rip you off.”

While many people opt to simply ignore “Check Engine” lights, this can cause bigger, more costly problems later. So the problem you could have fixed for a few hundred dollars turns into a repair of the catalytic converter, which would be over a thousand.  Plus, fixing such a problem provides another benefit: peace of mind.

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