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What Does An O2 Sensor Do? What Every Mechanic Should Know

Mechanics understand that engine parts have become complicated with sensors and computers running the show. The O2 sensor is one of those parts that still causes trouble for mechanics, and some wonder what does an O2 sensor do?

The O2 sensor is an integral part of any engine manufactured after 1980 when engine controls became electric. The O2 sensor monitors oxygen levels in the emissions and sends constant updates to the vehicle’s main computer.

When emissions levels get too high, the O2 sensor will force the car to make changes to optimize the emissions mixture.

What O2 Sensors Monitor

Most vehicles have more than one O2 sensor. If one of them does not work correctly, the central computer will turn on the check engine light. The O2 sensors are continually monitoring and altering the emissions based on several factors, including:

  • Engine workload
  • Engine temperature
  • Air temperature
  • Altitude
  • Barometric pressure
  • Gas to oxygen mixture

Oxygen Sensor Locations

Along with asking what does an O2 sensor do, people wonder about their location, too. New vehicles have at least two sensors. One is close to the engine, on the exhaust pipe at the beginning of the emissions system. The second is closer to the end of the emissions system near the muffler and catalytic converter.

Having O2 sensors in at least two spots gives the engine accurate information so the sensors can fine tune the gas-to-oxygen mixture. If a vehicle has a dual exhaust system, then the car will have a pair of sensors in each exhaust pipe.

The Lifespan of an O2 Sensor

As O2 sensors are in their fourth decade of existence, they have longer lifespans than the earlier versions. Older models lasted 60,000 miles, but new ones will run for 100,000 miles or more. The latest versions are designed to withstand high temperatures, rough roads, and other severe conditions.

When an O2 Sensor Begins to Fail

It is difficult to tell if an O2 sensor is going bad unless you have a diagnostic code reader. However, there are a few clues that could suggest the sensors is failing. One is that your vehicle’s gas mileage decreases without any other explanation. Performance tends to drop off, too.

Along with the check engine light turning on, another possible sign that the O2 sensor is on its last legs is a rough running car. Some people will attribute that to spark plug problems, but if the vehicle is still running rough, it could be the oxygen sensor.

Finally, if the exhaust is black and the spark plugs have issues, the oxygen sensors could be failing.

Why do O2 Sensors Fail?

Oxygen sensors fail as they age, but other reasons cause them to stop working correctly. If you put contaminated fuel in your car, the O2 sensors could fail sooner than expected.

Contamination from road salt and other external chemicals can damage the O2 sensor. If your engine is burning oil, your car’s O2 sensors can be damaged, too.

Other Sensors in Vehicles

Along with O2 sensors, vehicles have several different sensors that affect the way that cars run. Some of the sensors will trigger the check engine light, and some will affect the performance of the vehicle.

Mechanics should know what does an O2 sensor do and they should know what these other sensors do, too.

Tire Pressure Monitor Systems

If you have a model year 2006 or newer, your car has notified you that the tires need air. Tire pressure sensors will trigger a dashboard light when the air pressure drops at least 25 percent. Newer model vehicles have sensors in the valve stems, and older models have the sensors in the center of the wheel.

Tire pressure sensors use a battery that lasts five to seven years. You can replace the sensors when they fail, and many people have the procedure done when they get new tires. Some sensors stop working when people inject tire sealant into their tires.

Temperature Sensors

Another sensor that commonly shows up on the dashboard is the temperature monitor. This sensor affects the fuel mixture by monitoring the changes in air temperature and air density. If the sensor is not working correctly, the vehicle’s performance will be affected.

Emissions and fuel usage increase if the temperature sensor isn’t functioning correctly.

Mechanics can use a diagnostic reader to determine if the temperature sensors are functioning or not. Your vehicle’s heating and air conditioning use a temperature sensor, too. Since this sensor does not affect the way the car runs, if it breaks, it doesn’t trigger the check engine light.

You’ll notice the HVAC system doesn’t work well if the sensor breaks.

Fuel Temperature Sensor

The temperature of the fuel is a vital reading that keeps the car running efficiently. This sensor not only monitors fuel temperature, but it also uses data to inject the right amount of fuel based on its temperature.

At cold temperatures, fuel does not burn as efficiently, but if fuel is too warm, it burns faster. The sensor works to keep the fuel at the ideal temperature for the conditions. If the sensor begins to fail, car parts can be damaged from the car running out of gas.

Coolant Sensor

Coolant is an essential fluid in any vehicle. Without coolant, the car will overheat. To be sure that the vehicle runs at an appropriate temperature, coolant sensors monitor fuel consumption and engine emissions.

If the coolant sensor fails, the cooling fan stops working. Failure causes the engine to overheat. Ironically, the coolant sensor can be damaged when the engine overheats. If this happens, the coolant sensor needs to be replaced.

When a mechanic checks the coolant sensor with a diagnostic reader, the temperature of the engine should be the same as the air inlet temperature. The two should increase as the car warms up.

Vehicle Speed Sensors

Newer cars also have speed sensors, which are magnetically based and are on the transmission. These sensors monitor the speed of the vehicle, and they work with the antilock brakes which have wheel speed sensors, too. The transmission has other sensors that work with the speed sensors, also.

When the speed sensors fail, it is wiring problems that create the problem. Failure also happens when iron particles interfere with the magnets in the sensors. If the anti-lock brakes system isn’t working, then the sensor could be bad.

Throttle Position Sensors

This sensor system monitors the throttle, and a scan tool will show the throttle opening if the sensor is functioning correctly. This sensor is responsible for controlling the opening on the accelerator, so airflow matches the engine load.

You might not know what a throttle sensor is, but you might recognize when it is functioning poorly if you notice that your car has a hiccup during acceleration. It is common for throttle position sensors to show signs of wear at the idle position, which is why cars hesitate as miles build up.

Manifold Absolute Pressure Sensor

If you aren’t a mechanic, this sensor might be in part of the car you don’t know. This sensor watches the outside atmosphere and the intake vacuum to make decisions about the engine load. Some vehicles use a fuel injection system that requires data from the manifold sensor and the throttle sensor.

If this sensor is not working well, the results are similar to the throttle sensor as the motor will hiccup and hesitate. The sensor will make mistakes with spark timing and fuel, and air mixtures. A diagnostic scan tool can measure whether or not the manifold sensor is working correctly.

Position Sensors

Two position sensors work together to maintain the speed and firing in the engine. The crankshaft position sensor maintains the position and speed of the crankshaft, and the camshaft position sensor supports the engine’s firing order. If either of the sensors fails, the engine will not run.

How Sensors Work Together

Before cars had sensors, mechanics could turn a few screws and change a few filters to get their vehicles to run better. Now, all of the sensors work together to keep all of the systems in check.

They all feed into the main computer that prevents the car from failing.

Feeding into the Main Computer

These sensors continuously feed data into the main computer through a collection of modules. The sensors feed data to the modules which send them to the central computer, which then makes decisions about fuel-air mixtures, wheel speed, and so much more.

Without the sensors doing their jobs, the car would not be able to run efficiently.

Maintaining the Sensors

person touching gray rod on vehicle engine

Unfortunately, when the sensors go bad, they can be expensive to replace. It is better to replace them before they fail entirely. Waiting too long can actually do damage to the vehicle, especially related to performance and fuel efficiency.

In some cases, sensors need to be cleaned, which is much less expensive than having to replace them.

Keeping the sensors working correctly is key to keeping your car functioning properly. The lines of communication need to stay open to extend the life of your vehicle.

Well trained mechanics know how to care for sensors, modules, and main computers so your car can last a long time.