What happens when downstream oxygen sensor goes bad?

What Is An Oxygen Sensor?

An oxygen sensor is a small device located in your vehicle’s exhaust system. Its shape and size resemble that of a spark plug. Depending on its placement in regard to the catalytic converter, it can be located upstream (before the converter) or downstream (after the converter).

Most vehicles manufactured after 1990 have both upstream and downstream oxygen sensors. And dual-exhaust vehicles have a total of 4 O2 sensors.

What Does An Oxygen Sensor Do?

The role of the oxygen sensor is to monitor the amount of oxygen in the exhaust. This is unburnt oxygen that was initially injected into the fuel for proper combustion. Through a voltage signal, the O2 sensor communicates to the car’s computer how much oxygen is in the exhaust, so that the computer can adjust the fuel/oxygen mixture delivered to the engine.

The sensor placement before and after the catalytic converter allows it to keep track of the cleanliness of the exhaust, as well as monitor the efficiency of the converter.

How Does An Oxygen Sensor Go Bad?

The oxygen sensor in modern cars can last up to 100K miles, but typically you would experience problems sooner than that.

Over time, an oxygen sensor may become caked with byproducts of combustion, such as sulfur, lead, fuel additives, oil ash, etc. This contamination causes the sensor to lose its ability to produce voltage and send the right signal.

Using fuel that is not recommended for your vehicle or using low-quality gasoline may also speed up the oxygen sensor failure. And if you are skipping maintenance, especially things like timely spark plug and air filter replacement, you are increasing the likelihood of incomplete fuel combustion, which in turn leads to more dirt and grime in your emissions system.

What Are The Signs Of A Bad Oxygen Sensor?

In most cases, a bad oxygen sensor will trigger a check engine light. P0138 and P0135 are some of the codes you may expect to see on the OBD II reader, if you have one. Other than that, it’s difficult to spot a failing oxygen sensor.

It will inevitably lead to decreased gas mileage, but it’s usually not drastic enough for an average driver to notice. The decrease is gradual and happens over time, so unless you are keeping tabs on your MPGs, you will likely miss these signs. A bad or failing O2 sensor can also cause you to fail your emissions test.

How Much Does It Cost To Replace An Oxygen Sensor?

It will typically cost between $200 and $400 dollars to replace a single oxygen sensor. Most of it is the cost of the sensor itself, which can run anywhere from $30 to $200 depending on the year, make and model of your car. The cost of labor will depend on how accessible the sensor is in your specific vehicle.

In some cases, a bad oxygen sensor may indicate other problems, such as a failing catalytic converter, in which case the repair can get expensive.

This will vary from vehicle to vehicle, but here is a quick trick to determine how many your car has.

For each exhaust pipe your vehicle has, you will have one catalytic converter (a part that turns harmful gasses into less harmful gasses). For each catalytic converter, you will have two oxygen sensors. Many vehicles  will have only one catalytic converter, and therefore, two oxygen sensors, but some can have four or more. 

Most O2 sensors are the switching type that generate a rich or lean voltage signal depending on how much unburned oxygen is in the exhaust. A few, like the “titania” O2 sensors in some older Jeep, Nissan and Toyota applications change resistance to indicate a rich or lean condition in the exhaust.

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Oxygen (O2) sensors are part of the engine management system and are used to perform two important functions. The “upstream” O2 sensor(s) in the exhaust manifold(s) monitor the air/fuel mixture, while the “downstream” O2 sensor(s) to monitor the catalytic converter(s).

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Most O2 sensors are the switching type that generate a rich or lean voltage signal depending on how much unburned oxygen is in the exhaust. A few, like the “titania” O2 sensors in some older Jeep, Nissan and Toyota applications change resistance to indicate a rich or lean condition in the exhaust. In many late-model applications, an “Air/Fuel” sensor (which is actually a “wide band” O2 sensor) is used to monitor the fuel mixture.

On the fuel management side, the engine computer (Powertrain Control Module or PCM) uses the input from the upstream O2 sensor(s) to readjust the fuel mixture as needed for optimum emissions, fuel economy and performance. Input from the downstream O2 sensor(s) is used to detect any problems with the converter(s), and to fine-tune fuel trim.

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If a fault occurs in an O2 sensor or its internal heater or wiring circuit, the OBD II system should detect the problem, set one or more fault codes and turn on the Check Engine light. If diagnosis confirms a bad sensor (and not a wiring fault or some other type of engine problem that is affecting the sensor reading), the sensor needs to be replaced.

If one upstream O2 sensor in a high-mileage vehicle with a V6 or V8 engine has failed, chances are the other upstream O2 sensor on the opposite cylinder bank is probably reaching the end of its service life too. Replacing both sensors is recommended to restore like-new performance and fuel economy.

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 Downstream O2 sensors tend to be longer-lived because they are behind the converter and exposed to less heat. On the other hand, their undercar location exposes them to more road splash and corrosion. Because of this, a new downstream O2 sensor should be installed if the catalytic converter is being replaced.

Replacement oxygen sensors are available in two versions: exact fit and universal. Exact fit O2 sensors come complete with a wiring harness and connector that plugs into the engine wiring harness the same as the original sensor. Exact fit are fast and easy to install because they do not any require cutting and splicing of wires. Universal O2 sensors, on the other hand, do require some additional installation effort but allow fewer part numbers to provide much broader application coverage.

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Related sales opportunities for customers who are replacing an O2 sensor include a special oxygen sensor removal socket with a cutout for the sensor wire, antiseize compound for the sensor threads (if not preapplied to the sensor), and crimping pliers for joining wires on a universal O2 sensor.

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