How to Fix Your Car's Oxygen Sensor

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, according to CarMD.com, which sells an automotive diagnostic tool and provides repair information. The oxygen sensor unseats the formerly most common check engine light culprit: a loose gas cap. There are fewer reports of that problem because savvy motorists have learned to fix it themselves and consumers now buy new cars with capless gas tanks.

But don't despair. Replacing your car oxygen sensor will keep you from wasting money by burning extra gas, and the repair isn't horribly expensive. We know this firsthand. We had to replace the O2 sensor on our 1996 Lexus ES 300, the subject of our Debt-Free Car project, and it wasn't as much of a hassle or expense as we had feared.

After the dreaded check engine light appeared in our Lexus, we plugged the CarMD device into the car's computer to read the error code. In our case, the code was P0135, which indicated the oxygen sensor in bank 1 was malfunctioning. It was surprising to learn that something was wrong with the car since it still seemed to be running fine.

Even though a car seems to be behaving normally, a faulty oxygen sensor will cause the engine to start "gulping down gas," says Kristin Brocoff, director of corporate communications for CarMD.com. She says this problem can cause up to a 40% reduction in fuel economy. Sure enough, when we checked our fuel record for the driving we did while the check engine light was on, our mpg had taken a hit.

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, according to Bosch, a leading manufacturer of auto components. 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.

After we plugged CarMD's diagnostic device into the Lexus' onboard computer port, we connected it to our desktop computer. It accessed a database of information about this engine code and how to have it repaired. Among other things, the report included an average estimate just to buy a new oxygen sensor: $168.82.

At the first sight of a check engine light, most owners of new cars that are still under the factory warranty would simply make a beeline for the dealership's service bay. But car owners on a budget might want to go the do-it-yourself diagnosis route to save money. By using the CarMD device, or any engine code reader, drivers can learn what the problem is, and the skill level required to fix it, before attempting the task.

Modern cars have two to four oxygen sensors, Nielsen says. A V6 engine, such as the one in our Lexus, has one sensor in each exhaust manifold and one after the catalytic converter. 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.

Nielsen says that while a code reader might indicate that the problem is the car oxygen sensor, other problems can trigger the identical code — a disconnected vacuum hose will do it, for example.

As a first step, a car owner can look under the hood to see if there are any wires or hoses disconnected, Nielsen says. 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 what Nielsen calls "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."

In our case, we learned that the faulty O2 sensor was in the rear of the engine and difficult to reach, so the fix seemed above our skill level. Instead, we took the Lexus to Overseas Garage, in Long Beach, California. There, the mechanic told us that the new sensor would cost $117, plus $144 in labor for a total of $261. This was close to the $246 average cost cited by CarMD's Brocoff.

While many people opt to simply ignore "Check Engine" lights, Brocoff says 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."

Driving back from the garage, it was a relief not to stare at the glowing check engine light. This made us realize that fixing such a problem provides another benefit: peace of mind.