Symptoms and Causes of Bad O2 Sensors

Oxygen sensors are important emissions-related parts that sit within a cars exhaust system. All cars made after 1980 will have at least one oxygen sensor within the exhaust system. While O2 sensors don’t normally cause too many performance issues, a bad oxygen sensor can throw a check engine light and cause you to fail an emissions test. In addition to failed emissions and check engine lights, it can also cause poor fuel economy. Therefore, diagnosing and fixing bad oxygen sensors is important to keep your engine running healthy. This guide is going to discuss what oxygen sensors are and how they work and the common symptoms of bad oxygen sensors so that you can diagnose and repair a failed O2 sensor.

_A Guide to NOx Sensors and What You Need to Know 2

What is an oxygen sensor?

Oxygen sensors, or O2 sensors, sit within your cars exhaust system and measure the amount of oxygen within the exhaust gases produced by the engine. The point of this is to calculate air to fuel information which is then passed back to the engine control module.

Most older vehicles simply use the measurements for monitoring and simply throw a check engine light if the readings are out of range. However, newer vehicles with electronic fuel management will actually use these readings to adjust fuel delivery. Hence why a bad sensor can throw off your gas mileage.

O2 sensors have a heating element inside them. They must get up to operating temperature before they can properly measure oxygen levels. The heating element helps them get up to temp quickly after starting your vehicle. Multiple parts of the O2 sensor can fail such as the heating element or the wiring or plug.

How many O2 sensors does a car have?

Cars will have at least two oxygen sensors, with some having 4 or more. Each catalytic converter has an upstream and a downstream sensor. Upstream sensors sit before the catalytic converter and measure the exhaust gases before they are treated by the cat. Downstream sensors sit after the catalytic converter and essentially measure how good of a job the catalytic converter is doing at decreasing emissions.

Cars manufactured from 1981-1995 are required to have at least one oxygen sensor. Cars made after 1995 are required to have both an upstream and downstream sensor. Upstream sensors usually are located in the exhaust manifold or cylinder block, and downstream sensors sit within the catalytic converter piping.

Because oxygen sensors can measure the effectiveness of the catalytic converter, they are also a good indication of whether or not your cats are working properly.

Symptoms of Bad O2 Sensors

Symptoms in older cars will usually be less noticeable as the O2 sensors don’t impact fuel delivery. Therefore, you will likely just get a check engine light and a fault code. Newer cars with more advanced fuel delivery systems will notice more symptoms. A bad sensor can cause the fuel system to either send more or less fuel to the engine, which can then impact performance, idling, and driveability.

1) Check engine light

Bad O2 sensors will always throw a check engine light. A P0141 engine code is a frequent fault code for failed oxygen sensors and is usually associated with the downstream sensor. Other common engine codes are P015x and other P014x codes, depending on how many sensors you have.

Reading the fault code is as simple as plugging an OBD scanner into the engine. However, since something like a bad catalytic converter can also cause a check engine light for the O2 sensor, it isn’t always a telltale sign that the O2 sensor itself is bad.

2) Decreased fuel economy

Typically when the downstream O2 sensor goes bad, it results in increase fuel consumption. When the downstream sensor is bad it causes the engine to run rich and receive more fuel than is necessary, which causes fuel economy to decrease. This is going to be more noticeable on newer vehicles with electronically controlled fuel delivery.

3) Poor idling and misfires

Similar to the previous issue, a bad downstream sensor can cause idling issues, misfires, and some minor performance issues. As discussed, a bad sensor can cause the engine control module to think the engine is either running rich or lean when in fact it is running just fine. The fuel system will then adjust fuel delivery to compensate for the rich or lean conditions which can then cause poor idling, cylinder misfires, and various acceleration and performance related issues.

O2 sensors usually won’t cause any extremes. You can typically drive just fine on a bad O2 sensor. The misfires and rough idling will generally be pretty mild, but this depends on your vehicle.

4) Failed emissions tests

If you have to do an OBD emissions test, you will fail automatically if you have a fault code that indicates a bad O2 sensor. Even if it’s just a bad sensor you will still fail since the emissions test isn’t able to tell whether the issue is with the sensor or with the catalytic converter itself.

The primary goal of the O2 sensors is to measure emissions and make sure the cars emissions equipment are functioning properly. Therefore, if the O2 sensor is bad it is an automatic fail. However, this will only be noticeable if your emissions test includes an OBD hookup.

5) Bad catalytic converter

This is less of a symptom and more of a cause of bad O2 sensors. The challenge is that it can be difficult to diagnose whether the cat itself is bad or if the sensor is bad. A bad catalytic converter would throw off the readings from the downstream O2 sensor. This happens because the failed cat won’t reduce any of the emissions in the exhaust gas, which then triggers a bad reading from the downstream sensor, throwing a CEL.

Bad O2 Sensors Summary

O2 sensors are critical emissions and fuel related sensors. A bad O2 sensor can cause you to fail an emissions test in addition to cause MPG decreases, poor idling, and decreased performance. Most new vehicles have at least 4 oxygen sensors which can make it difficult to distinguish a bad sensor from a failed catalytic converter, or other issue.

A bad oxygen sensor on old cars likely won’t result in much other than a check engine light and some fault codes. On newer cars you can expect bad gas mileage, poor idling, and failed emissions tests. Common fault codes for downstream O2 sensors a P0141, but range from P014x to P015x codes depending on your vehicle.

One of these engine codes can also mean a bad catalytic converter, but O2 sensor failure is a lot more prevalent than catalytic converter failure.