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10 Bad O2 Sensor Symptoms (What to Look Out For)

The oxygen sensor communicates air-fuel ratio measurements inside a car’s engine to its PCM/ECU to adjust accordingly. It also accounts for barometric pressure, altitude, and ambient/engine temperature (among other things) to determine whether the engine burns a lean or rich mixture.

When an O2 sensor goes bad, it can no longer trigger accurate fuel injection levels, regulate exhaust gas, or ensure efficient fuel combustion. Consequently, the vehicle may release harmful environmental pollutants or carbon-based compounds – in addition to having subpar engine performance.

Because of how vital an oxygen sensor’s role is in overall vehicle performance and emissions, spotting bad O2 sensor symptoms early on has become a necessary skill for drivers. A persistent Check Engine Light, intermittent stalling, or poor gas mileage are just a few of the signs to watch out for. So, what are the symptoms of a bad O2 sensor?

Here are 10 common bad oxygen sensor symptoms:

  1. Continually failed emission tests
  2. Flashing or illuminated CEL (Check Engine Light)
  3. Misfiring, rough idling, or stalling
  4. Bad fuel economy/gas mileage
  5. Poor engine performance
  6. Engine noises
  7. Black exhaust fumes
  8. Sulfuric/rotten-egg smell from the exhaust
  9. Sudden catalytic converter failure
  10. Engine overheating

Never put off inspecting and replacing your vehicle’s O2 sensors if you experience any of these problems. These devices are typically located along the exhaust system or between the exhaust manifold and the catalytic converter.

Faulty O2 sensors take at most 30 minutes to repair (including an ECU reset) and are pretty accessible. Should you feel uneasy replacing them on your own, seek the help of a professional mechanic or a vehicle owner who is more experienced.

Bad O2 Sensor Symptoms

Rusty Bad O2 Sensor

1. Continually Failed Emission Tests

Functionally, an O2 sensor is an emissions device designed to reduce air contaminants emitted from the tailpipe by regulating a vehicle’s air-fuel mixture.

These sensors are crucial for passing emissions testing, which is a general requisite for completing one’s vehicle registration (although some states and non-U.S. locations do not require passing these tests).

While it is common for some car owners not to pass this checkpoint on the first try, repeatedly failing emissions testing may already indicate that something is amiss with one or all of your O2 sensors.

2. Flashing or Illuminated CEL (Check Engine Light)

Among the usual triggers of the Check Engine Light going off is a defective O2 sensor. Yet unknown to many, it is an indirect cause – meaning faulty O2 sensors cannot trigger the CEL on their own but can cause other vehicle systems to fail and the Check Engine Light to illuminate.

Because a glowing CEL can mean just any engine part malfunction, it is best to run an OBDII Diagnostic Scanner (view on Amazon) and try to obtain error codes that will point you towards the specific system or component at fault.

Examples of error codes that usually show in recent vehicle models are P0172 (System Too Rich), P0136 (Heated Oxygen Sensor Circuit Malfunction), and P0131 (O2 Sensor Circuit Low Voltage). Otherwise, the correct diagnosis of this issue will require contacting your local mechanic.

3. Misfiring, Rough Idling, or Stalling

An O2 sensor in good shape will never allow the oxygen-fuel ratio in your car to be thrown off, as this device controls engine timing and combustion intervals necessary for effective combustion.

So, if you notice frequent misfiring, rough idling, or any other irregularity in engine function along these lines, it is safe to conclude you are experiencing bad downstream O2 sensor symptoms.

Other problems expected from failing oxygen sensors are power loss, engine hesitation, and stalling when starting the car. A vehicle feeling sluggish for no apparent reason is also a telltale sign.

Of the three indicators mentioned, rough idling is probably one of the first bad O2 sensor symptoms anyone driving a car will notice. It is usually followed by misfiring and, lastly, by vehicle stalling.

With the first two, your car can still power on at a lower level than when its engine runs optimally. But with stalling, occurrences of misfiring have either been ignored too long or worsened to the point where the engine can no longer sustain itself with remaining working pistons in a vehicle’s cylinder block.

Even at this stage, it is hard to tell if faulty O2 sensors are directly or indirectly causing these problems.

When narrowing down the problem source, replacing your car’s spark plugs first can help. But if you suspect fuel starvation as the cause of stalling or starting issues, rule it out by performing a fuel pressure test.

Doing so helps determine if problems exist with the fuel pressure regulator, injectors, or other fuel system components. Proceed with replacing bad O2 sensors if all of these elements check out.

4. Bad Fuel Economy/Gas Mileage

Fuel consumption does increase slowly over time, especially for older, carbureted vehicles, and is usually caused by worn-out engine components. But if it occurs abnormally or shortly after having new oxygen sensors installed, chances are your vehicle’s air-fuel mixture is either running lean or rich.

Customarily, the next step to resolving this issue is by adjusting the car’s carburetor settings or replacing a loose vacuum hose as needed. But with fuel-injected, computer-controlled vehicles, this drastic drop in fuel economy is part of bad downstream O2 sensor symptoms and warrants a replacement, even for cars equipped with Denso oxygen sensors (view on Amazon).

5. Poor Engine Performance

Combustion difficulties caused by bad O2 sensors and an imbalanced air-fuel ratio result in weak engine performance. Typically, this performance drop is preceded by episodes of misfiring, intermittent idling, or stalling.

Sometimes, these bad O2 sensor symptoms disappear when your vehicle starts moving – however, do not be appeased by this.

When not addressed early on, it could be accompanied by or escalate to sputtering, speed stasis, restricted acceleration or engine hesitation, power surges, or, worse, loss of power. So, make sure not to tarry on that much-needed trip to the mechanic.

6. Engine Noises

Because faulty oxygen sensors fail to function as a proper emissions device, a vehicle ends up with excessive amounts of carbon deposits in the combustion chambers due to the lack of air-fuel mixture control. In turn, this results in a lean mixture causing engine knocking/pinging noises and pre-ignition, among other things.

This symptom is quite vague and does not necessarily mean a bad O2 sensor right away (it usually points to a dirty carburetor or need for top-end work). However, it will be worthwhile to check on the condition of the sensors, especially when the noises occur during idle.

7. Black Exhaust Fumes

Black Exhaust Fumes From Car Exhaust

With bad O2 sensors, the air and fuel quantities inside your engine become imbalanced since the devices meant to regulate air intake and fuel delivery no longer work the way they should.

Ultimately, this adversely affects combustion efficiency, leading to unburned fuel, backfiring, or soot-like smoke emitted from the vehicle’s exhaust – alongside high fuel consumption, poor idling, and hard-starting issues.

8. Sulfuric/Rotten-Egg Smell From the Exhaust

A heavy gasoline or sulfuric smell coming from the tailpipe usually goes with black smoke residue, both of which are bad O2 sensor symptoms. But there may be cases where they signal a problem with the vehicle’s fuel system or injectors.

Either way, this distinct smell means there is excess fuel inside the engine and that the air-fuel mixture of the vehicle needs to be corrected. To side with caution, better perform troubleshooting steps addressing both fuel system and oxygen sensor problems – this way, you can pinpoint what is causing the putrid smell.

9. Sudden Catalytic Converter Failure

Catalytic converters are also part of a vehicle’s emissions system, similar to O2 sensors. They work by alternating between rich and lean mixtures and, in doing so, control the amount of air in the exhaust – hence, reducing toxic gases released into the atmosphere.

Although it comprises a major part of a car’s emissions system, it largely relies on O2 sensors to function properly. So naturally, a catalytic converter will incur damage (to the point of complete failure) if O2 sensors malfunction and send the wrong readings to the PCM.

You will know one or all of your catalytic converters has gone awry if you encounter any of these symptoms:

  • Compression misfiring caused by leaky valves or head gasket
  • Corrosion or physical damage
  • Fuel contamination
  • Internal coolant leaks due to cracks in the head gasket
  • Ignition misfiring caused by a fouled spark plug or shorted plug wire
  • Oil burning due to worn valve guides, seals, rings, or cylinders

10. Engine Overheating

Consistent engine overheating is one of the less common bad O2 sensor symptoms. It is more popularly associated with problems regarding a vehicle’s top-end or electrical system.

But in the rare instance that it pertains to O2 sensor issues, overheating could only mean one thing – the car owner dismissed early signs of bad O2 sensors to the point where the sensors are extremely worn, and a replacement is well overdue.

Despite the risks that a disregarded overheated engine brings with it, some drivers (against their better judgment) push the limits of their stock O2 sensors. Unfortunately for this symptom, it will not go away until such time the defective sensors are replaced.

Causes of Premature O2 Sensor Failure

  • Carbon buildup from the engine, grime, dirt, or any other kind of debris that can get into the air intake and fuel systems can cause damage to O2 sensors.
  • Physical damage to the area O2 sensors are located (along the exhaust system, exhaust manifold, or catalytic converter).
  • Low-quality fuel, fuel contaminants, or the engine burning excessive oil also cause O2 sensors to get clogged and stop measuring fuel delivering accurately.
  • Skipping on scheduled maintenance or periodic change of spark plugs, air filters, fuel filters, and the like can lead to O2 sensor damage, eventually translating into combustion inefficiencies.
  • Using fuel additives for extended periods can unfavorably influence the vehicle’s air-fuel mixture and exhaust fumes.
  • Utilizing an incorrect or lower fuel octane rating than what is advised.
  • Not adhering to the manufacturer-recommended oxygen sensor replacement schedule.
  • O2 sensors become caked or gunked due to lack of vehicle upkeep.
  • Leaving early signs of bad O2 sensors unattended.
  • Not having a full-blown vehicle inspection at least once a year.

Cost of Replacing an O2 Sensor

The estimated oxygen sensor replacement cost depends on a vehicle’s year, make, and model, plus labor fees if you have a professional mechanic doing the job. Brand-new, an oxygen sensor costs $30 to $300 each.

Meanwhile, hourly labor rates range from $40 to $200 but may still change based on the number of sensors needing replacement, the difficulty of accessing these emissions devices, and the garage or mechanic where you bring your car.

Thankfully, replacing O2 sensors is simple and takes an average of half an hour or $100 for a skilled professional with the right tools to perform. These figures would be true, barring complications to the vehicle’s emissions system.

If not severely worn out, the cost of an oxygen sensor replacement would occupy the lower end of the repair expense spectrum. But taking into account other components that fall under the same category as O2 sensors, vehicle owners may collectively be looking at shelling $500 to $2,500 (including parts and labor) out of their pocket.

How Long Do O2 Sensors Last?

There are a ton of resources online with varying responses to this question. Summing up these references, O2 sensors can last between 30,000 and 100,000 miles. But again, the longevity of oxygen sensors depends on whether a vehicle is new or has seen better days.

Vehicles manufactured in the last 15 years are observed to have O2 sensors lasting 30,000 to 50,000 miles or three to five years before necessitating replacement. Conversely, O2 sensors in modern cars have sensors built to last 60,000 to 100,000 miles or seven to ten years.

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

To continue driving your vehicle despite acknowledging you have bad O2 sensor symptoms is counter-intuitive. Not only are you intentionally releasing harmful fumes into the air, but you are also running your vehicle to incur further engine damage that could have been avoided if you chose to replace bad sensors right away.

Delaying on replacing faulty sensors is not going to help save you money on repair costs. More importantly, you are endangering yourself on the road by opting to drive a car whose O2 sensors cannot appropriately monitor air intake and fuel delivery.

Conclusion – Bad Oxygen Sensor Symptoms

Oxygen Sensor O2 in the Exhaust Pipe

In summary, below are 10 common bad oxygen sensor symptoms:

  1. Continually failed emission tests
  2. Flashing or illuminated CEL (Check Engine Light)
  3. Misfiring, rough idling, or stalling
  4. Bad fuel economy/gas mileage
  5. Poor engine performance
  6. Engine noises
  7. Black exhaust fumes
  8. Sulfuric/rotten-egg smell from the exhaust
  9. Sudden catalytic converter failure
  10. Engine overheating

These problems can prove challenging to fix but are mostly preventable if addressed promptly. This non-exhaustive list should help you nip impending O2 sensor issues in the bud and – with added help from proper upkeep and regular inspection of your vehicle – make those oxygen sensors last a bit longer.