10 Symptoms of a Bad O2 Sensor: What to look out for.

10 Symptoms of a Bad O2 Sensor: What to look out for.

The oxygen sensor tells the PCM/ECU of a car how much air and fuel are in the engine so that it can make adjustments. It also takes into account things like barometric pressure, altitude, ambient, and engine temperature to figure out if the engine burns a lean or rich mixture.

When an O2 sensor breaks, it can’t trigger the right amount of fuel injection, control the exhaust gas amount, or ensure fuel is burned efficiently. As a result, the car may give off harmful pollutants or carbon-based compounds, and its engine may not work as well as it should.

Because of how important an oxygen sensor is to a vehicle’s overall performance and emissions; drivers need to be able to spot early signs of a bad O2 sensor. Some signs to look out for are: a Check Engine light that stays on, a car that stalls sometimes, or a car that gets bad gas mileage. So, what are the signs that an O2 sensor is broken?

The following are 10 common symptoms of a bad oxygen sensor:

  1. Concurrent failed emissions testing
  2. CEL that flashes or illuminates (Check Engine Light)
  3. Misfires, erratic idling, and stalling
  4. Poor fuel economy/gas mileage
  5. Poor engine efficiency
  6. Engine noises
  7. Black exhaust fumes
  8. Sulfurous/rotten-egg stench emanating from the exhaust
  9. Unexpected catalytic converter breakdown
  10. Engine Overheating.

If any of these things happen, you should immediately check and replace your car’s O2 sensors. These devices are usually placed along the exhaust system or between the catalytic converter and the exhaust manifold. Faulty O2 sensors can be fixed in no more than 30 minutes, even with an ECU reset, and are easy to get to. If you are not comfortable doing it yourself, ask a professional mechanic or another car owner who has done it before to help you.

Symptoms of a Bad O2 Sensor

  1. Concurrent failed emissions testing.

An O2 sensor is a type of emission device that controls the mixture of air and fuel in a car’s engine so that less pollution is released from the tailpipe. These sensors are necessary to pass the emissions test, which is usually a requirement for registering a car.

Some car owners don’t pass this checkpoint on the first attempt, but if you fail the test more than once, it could mean that something is wrong with one or more of your O2 sensors.

  1. CEL that flashes or illuminates (Check Engine Light)

A broken O2 sensor is a common reason why the Check Engine Light comes on. However, many people are unaware that it is an indirect cause, which means that faulty O2 sensors do not cause the CEL to illuminate on its own, but can cause other car systems to fail and the CEL to illuminate.

A blinking CEL could mean that a part of the engine is broken, it is best to use an OBDII Diagnostic Scanner and try to get error codes that will tell you which system or part is broken. Examples of error codes that usually show in recent vehicle models include P0131 (O2 Sensor Circuit Low Voltage), P0172 (System Too Rich), and P0136 (Heated Oxygen Sensor Circuit Malfunction). If the problem persists, you need to talk to your local mechanic to figure out what’s wrong.

  1. Misfires, erratic idling, and stalling

If your O2 sensor is in good shape, it won’t change your car’s oxygen-to-fuel ratio. This is because the sensor controls the timing of the engine and the intervals between combustion that are needed for effective combustion. So, if you notice a lot of misfiring, rough idling, or any other strange engine behavior, you can be sure that your downstream O2 sensor isn’t working right. When oxygen sensors break, the car might lose power, the engine may stutter, and the car will struggle to start. Another sign is if the car feels slow for no apparent reason.

Out of the three signs cited, rough idling is probably one of the first signs that an O2 sensor isn’t working right. It is usually followed by the engine misfiring, and then the car stops moving. With the first two, your vehicle can still run, but not as well as when the engine runs at its best. When a car stalls, it’s because misfiring has either been ignored for too long or gotten so bad that the engine can’t keep running with the pistons in the cylinder block.

It’s hard to tell if bad O2 sensors are causing these problems directly or indirectly. When trying to find out what’s wrong with your car, you can start by replacing the spark plugs. If you think insufficient fuel is causing your car to stall or not start, you can rule that out by doing a fuel pressure test. This helps determine if there are problems with the fuel pressure regulator, the injectors, or other fuel system parts. If any of these things are truly the case, you should replace the bad O2 sensors.

  1. Poor fuel economy/gas mileage

Fuel consumption gradually increases with time, particularly in older, carbureted automobiles, and is typically caused by worn-out engine parts. If it happens out of the ordinary or soon after getting new oxygen sensors, it’s likely that your car’s air-fuel mix is either too lean or too rich.

Usually, the next step to fix this problem is to change the settings on the car’s carburetor or replace a loose vacuum hose, if necessary. However, in fuel-injected, computer-controlled cars, this sudden drop in gas mileage is a sign of a bad downstream O2 sensor and means the car needs some parts replaced, even if it has Denso oxygen sensors.

  1. Poor engine efficiency

Weak engine performance is caused by problems with combustion that come from faulty O2 sensors and an air-to-fuel ratio that is not balanced. Most of the time, misfiring, stalling, or short idling periods come before this performance drops.

Sometimes, these signs of a bad O2 sensor go away when your car starts moving, but don’t be reassured by this. If it isn’t fixed immediately, it could lead to sputtering, speed stagnation, slow acceleration, power surges, engine hesitation, or loss of power. So, don’t wait around when you need to go to the mechanic.

  1. Engine noises

Since faulty oxygen sensors don’t work as proper emissions devices, there are too many carbon deposits in a car’s combustion chambers. This is because the air-fuel mixture can’t be controlled. In turn, this makes the mixture too lean, which can cause the engine to knock or ping and sometimes  pre-ignition.

This sign is unclear and does not always mean that the O2 sensor is broken (it typically points to a dirty carburetor or a need for top-end work). Nevertheless, checking on the sensors is a good idea, especially if the noises happen when the car is not moving.

  1. Black exhaust fumes

The heavy stench of gasoline or sulfur comes from the tailpipe, and so does black smoke residue. Both of these are signs of a bad O2 sensor. Sometimes they mean something wrong with the car’s fuel system or injectors. Either way, this strong smell means too much fuel in the engine, and the car’s air-fuel mix needs fixing. To be safe, you should do troubleshooting steps for both the fuel system and the oxygen sensor. This will help you figure out what is causing the bad smell.

  1. Sulfurous/rotten-egg stench emanating from the exhaust

Like O2 sensors, catalytic converters are also a part of a car’s emissions system. They work by switching between rich and lean mixtures. This controls the air in the exhaust, which cuts down the amount of harmful gases released into the air. Even though it is a big part of a car’s emissions system, it mostly works because of the O2 sensors. So, it makes sense that a faulty O2 sensor that sends wrong readings to the PCM will cause damage to a catalytic converter, even to the point of total failure.

If any of the following occur, it means one or more of your catalytic converters are malfunctioning:

  • Compression misfiring due to leaking valves or a faulty head gasket
  • Corrosion or physical damage
  • Fuel contamination
  • Internal coolant leakage due to head gasket fissures
  • Misfiring ignition caused by a clogged spark plug or a damaged plug wire.
  • Burning oil as a result of worn valve guides, seals, rings, or cylinders.
  1. Engine Overheating

One of the least common signs of a bad O2 sensor is that the engine keeps getting too hot. People usually think of it as problems with a car’s engine or electrical system. However, when it comes to O2 sensor problems, which is rare, overheating can only mean one thing: the car owner ignored early signs of bad O2 sensors until the sensors were so worn out that they needed replacement.

Even though an overheated engine is dangerous when you don’t take care of it, some drivers push the limits of their stock O2 sensors when they should know better. The problem with this symptom is that it won’t go away until the broken sensors are replaced.

Some Causes of Premature O2 Sensor Failure

  • O2 sensors can be damaged by engine carbon buildup, grime, dirt, and other debris entering the air intake and fuel systems.
  • Physical damage to the location of the O2 sensors (along the catalytic converter, exhaust system, or exhaust manifold).
  • Low-quality gasoline, fuel impurities, or an engine that consumes excessive oil might clog O2 sensors and prevent them from measuring fuel delivery effectively.
  • Skipping planned maintenance or the periodic replacement of spark plugs, air filters, and fuel filters, among other components, can result in O2 sensor deterioration, leading to combustion inefficiencies.
  • Extended use of gasoline additives can negatively affect the vehicle’s air-fuel mixture and exhaust emissions.
  • Using fuel with an inadequate or lower octane grade than recommended.
  • Not following the manufacturer-recommended replacement schedule for the oxygen sensor.
  • O2 sensors become smudged or clogged due to a lack of vehicle maintenance.
  • Ignoring early indicators of faulty O2 sensors.
  • Not getting a comprehensive vehicle inspection annually.

Cost of Fixing a Bad O2 Sensor

The estimated cost to replace an oxygen sensor depends on the car’s year, make, and model, plus labor costs if a professional mechanic does the job. Each new oxygen sensor costs between $30 and $300. The labor cost per hour ranges from $40 to $200, but this can still change depending on how many sensors need replacement, how hard it is to get to the emissions devices, and which garage or mechanic you take your car to.

Luckily, replacing O2 sensors is easy and only takes a skilled professional with the proper tools about 30 minutes and maybe $100. This should be the standard unless there are problems with the car’s emissions system.

If the oxygen sensor isn’t too worn out, the cost to replace it would be on the lower end range of repair costs. However, if you consider other parts in the same group as O2 sensors, car owners may have to pay between $500 and $2,500 (including parts and labor) to fix their cars.

Frequently Asked Questions

Is It Harmful to Drive a Car with a Bad Oxygen Sensor?

It doesn’t make sense to keep driving your car when you know the O2 sensor is bad. Not only are you putting harmful gases into the air on purpose, you are also doing more damage to the engine that could have been avoided if you had replaced the bad sensors immediately. You might spend more on repairs if you wait to replace broken sensors. More importantly, you are putting yourself in danger on the road if you drive a car whose O2 sensors can’t monitor fuel delivery and air intake properly.

How Long Do O2 Sensors Last?

There are a lot of different answers to this question online. However, we can say that O2 sensors can last between 30,000-100,000 miles. But again, how long oxygen sensors last depends on whether a car is brand new or has been around for a while. O2 sensors in cars made in the last 15 years last between 30,000 and 50,000 miles, or 3-5 years, before they need to be replaced. On the other hand, modern cars have O2 sensors that are made to last between 60,000 and 100,000 miles or 7-10 years.


In summary, the following are 10 common symptoms of a bad oxygen sensor:

  1. Concurrent failed emissions testing
  2. CEL that flashes or illuminates (Check Engine Light)
  3. Misfires, erratic idling, and stalling
  4. Poor fuel economy/gas mileage
  5. Poor engine efficiency
  6. Engine noises
  7. Black exhaust fumes
  8. Sulfurous/rotten-egg stench emanating from the exhaust
  9. Unexpected catalytic converter breakdown
  10. Engine Overheating.


These problems can be hard to fix, most of which can be avoided if they are dealt with quickly. This list isn’t all-inclusive, but it should help you catch any problems with your O2 sensors before they get worse. Proper maintenance and regular car inspections will also help your oxygen sensors last longer.