6 Signs Of A Bad O2 Sensors – You Should Know

Last Updated on February 5, 2024 by John Robinson

Your car’s oxygen sensors (also known as O2 sensors) are a crucial part of the exhaust system. They measure the amount of oxygen in your exhaust and send this information to your vehicle’s computer to make adjustments based on whether the air/fuel mixture is too rich or too lean. 

If your O2 sensor starts to degrade, all sorts of problems can result—including poor fuel economy and performance issues like rough idling or misfiring. So when do you know if your O2 sensor needs to be replaced? Read on for some signs of a bad o2 sensors that it may be time for new ones:

1. Your Engine Takes Too Long To Start

If your engine takes too long to start, or if it starts and then dies, this could be indicative of an O2 sensor problem. For example, if the engine cranks but doesn’t start or start and stalls immediately, this is also a sign that you should replace the O2 sensors on your car. The same goes for engines that start but stall soon after starting or frequently stalling.

This is often a problem with the O2 sensors, but it could also be a fuel injector problem. It’s worth checking the O2 sensors first, though. Once you decide that it’s sensors you need, you can head to the car supplies shop you trust, or if you are shopping online (which is more convenient overall), you can get them from https://www.dpfpartsdirect.com or another reputable seller.

2. Your Car Fails The Emissions Test

If you’re in a state with an emissions test, your car will likely fail the initial inspection. This is because O2 sensors measure and regulate the amount of oxygen in your exhaust gases. A faulty sensor can result in excess levels of pollutants such as carbon monoxide and unburned hydrocarbons being emitted into the air by your vehicle.

If you’re not required to pass a safety or emissions test but would like information on how well your car’s engine has been treated by its current O2 sensors, then this may be enough for replacement. However, if there are no other signs that these components need replacing, aside from their age, it might be best for them to stay put until further notice!

Measurement of exhaust emissions at the technical inspection station. A car repairman applies a probe to the exhaust.

3. A Glowing Check Engine Light

If your car’s check engine light comes on, it could be triggered by a faulty oxygen sensor. The code will differ depending on which sensor gives off the error.

In most cases, the check engine light is triggered by a bad O2 sensor. If this happens, you’ll notice that your car has decreased fuel economy or misfires while accelerating or idling. You may also see an increase in exhaust gas emissions, which could cause poor performance at higher speeds and even trigger a warning light for emission control system malfunction if you ignore it for too long!

4. You Have A Decrease In Acceleration

A decrease in acceleration is one of the most common signs that your O2 sensors need to be replaced.

O2 sensors play a crucial role in regulating air and fuel intake into the engine, which impacts how well your car can accelerate. For example, if your O2 sensors are not working correctly, this may cause your vehicle to struggle to get up hills or go over speed bumps.

When a car loses its ability to accelerate, it can become frustrating and dangerous on the road. However, suppose your vehicle is having trouble accelerating at all times. In that case, it is probably best to get this checked out immediately, so all passengers are safe during operation.

5. You Get A Rough Idle Or Your Engine Misfires

If your engine is misfiring, it could mean that you have a bad O2 sensor. However, there are several reasons why your car would misfire. If the problem persists after replacing the oxygen sensor and other parts of the system, it’s time to take it back to the shop for further inspection.

A few things that can cause an engine to misfire include bad spark plugs, fuel injectors, and fuel pumps.

6. You Have Increased Fuel Consumption

You should first check your fuel consumption. You can do this by looking at the gas gauge on your dashboard or observing how much you’re paying for gas. If you’ve noticed an increase in either one, then there’s a good chance that you may have a faulty oxygen sensor.

If you don’t want to rely on visual cues and would rather have more complex data regarding your car’s performance, consider downloading an app that tracks fuel efficiency and economy from the comfort of your smartphone. Also, remember that these apps are often so detailed that they include historical information about when it was the last time either criterion was met. If this information shows a trend of abnormally high efficiency or economy scores over time, it could indicate a problem with your O2 sensor(s).


O2 sensors can be a life-saving device for your car. While they are unlikely to cause any problems on their own, they can indicate an issue in the engine that could lead to significant damage or even an accident if left unattended. If you feel like one of these signs is happening with your vehicle, it’s time to get some new O2 sensors!

John Robinson
John Robinson

Howdy! I’m John Robinson from Levittown, New York. I am a mechanical engineer for 15 years and already had an established car repair company. I developed a personal relationship with cars since I was a kid. I always love the sounds and smell emitted by a car or truck and even at construction machinery. Since then I have been married but my love for cars only grew.