5 Symptoms of a Bad Oxygen Sensor (and Replacement Cost)

Your vehicle’s oxygen sensors monitor combustion efficiency, but like other types of sensors, they can eventually fail. Ignoring deteriorating oxygen sensors allows problems to multiply, hurting performance and fuel economy.

Learn to recognize the subtle symptoms of a bad oxygen sensor because if you catch it too late, you may be dealing with much more expensive repairs down the road.

Purpose of Oxygen Sensors

Oxygen (O2) sensors are small, yet extremely important parts of the much more intricate emissions systems found on today’s vehicles. These systems ensure that tailpipe emissions are kept to an absolute minimum, for environmental reasons.

A vehicle’s oxygen sensors are designed to measure the amount of unburnt oxygen present in a vehicle’s downstream exhaust. This data is relayed to a vehicle’s PCM/ECM, where it is used to compute vital fuel trim data for each respective engine bank. This process makes sure that combustion efficiency is optimized under all operating conditions.

Location

Most vehicles are equipped with at least two O2 sensors, mounted within the exhaust system. One of these sensors is found upstream of a vehicle’s catalytic convertor, while the latter is found downstream of the catalyst.

Vehicles featuring an engine of a “V” configuration typically come equipped with at least 4 oxygen sensors, two of which correspond to each engine bank.

Related: Bank 1 vs Bank 2 Oxygen Sensors

Lifespan

Generally speaking, the service life of the average oxygen sensor spans 40,000-80,000 miles. That being said, it’s not uncommon for the occasional O2 sensor to remain in service for in excess of 100,000 miles.

On the other side of the coin, some O2 sensors, especially cheap aftermarket varieties or those subjected to contamination of one type or another, fail in as little as 30,000 miles.

Bad O2 Sensor Symptoms

The presence of a faulty oxygen sensor is often accompanied by a host of secondary symptoms, some of which can prove rather noticeable in the long run. Learning to identify these signs can prove quite helpful in reaching a quick diagnosis.

#1 – Check Engine Light

One of the first symptoms of a faulty O2 sensor, in most cases, is a newly illuminated check engine light. This occurs when O2 sensor specific faults, such as P0130, P0141, P2270, etc., are actively logged by a vehicle’s PCM/ECM.

Since oxygen sensors are constantly monitored by your vehicle’s computer systems, this warning light serves as an early indication that something isn’t functioning within normal parameters.

#2 – Reduced Engine Performance

When one or more of a vehicle’s O2 sensors fail, it is not uncommon for engine performance to suffer. This is due to the fact that fuel trims are skewed toward less-than-optimal rates, as a failsafe measure of sorts.

Since oxygen sensors are designed to measure the amount of unburnt oxygen present in a vehicle’s downstream exhaust, their failure directly impacts the PCM/ECM’s ability to compute vital fuel trim data.

This disruption in the carefully balanced fuel management system can lead to noticeable performance issues such as rough idling, hesitation during acceleration, or overall sluggish engine response. The engine may run either too rich or too lean, as the computer can no longer make accurate adjustments based on exhaust oxygen content.

#3 – Bad Fuel Economy

It is also relatively common for a vehicle’s gas mileage to suffer when an O2 sensor failure is encountered. This occurs when a vehicle’s engine overfuels in response to incomplete or irrational O2 sensor feedback.

Without accurate oxygen readings, the engine’s fuel management system typically defaults to a rich fuel mixture as a protective measure. This means more fuel is being injected than necessary for proper combustion, leading to significant decreases in your car’s MPG (and your bank account).

See Also: 5 Symptoms of a Bad Air/Fuel Sensor

#4 – Changes in Exhaust Output

You might also notice that your vehicle’s exhaust output has become noticeably heavier, or darker in color. Additionally, this exhaust might have a distinct odor, that seems far more rich than normal.

These changes occur because the exhaust system, which normally relies on oxygen sensor data to maintain optimal air-fuel ratios, is now operating with incorrect fuel mixtures. The darker or black exhaust smoke is typically a sign of over-fueling, while the strong fuel smell indicates unburnt fuel in the exhaust stream.

These symptoms are particularly noticeable during acceleration or when the engine is under load. In severe cases, you might also detect a sulfuric or rotten egg smell, which could indicate that the catalytic converter is being affected by the improper fuel mixture.

#5 – Failed Emissions Test

Yet another symptom of a faulty oxygen sensor is the lack of ability to pass state or municipality-mandated emissions inspection. When an oxygen sensor fails, tailpipe emissions levels tend to exceed that which is permissible by today’s legal standards.

The vehicle’s emissions control system cannot properly regulate the air-fuel mixture, leading to increased emissions of harmful gases such as carbon monoxide and hydrocarbons.

In states where testing is mandatory, emissions test failure can prevent vehicle registration renewal and may require immediate attention to bring the vehicle back into compliance with environmental regulations.

Oxygen Sensor Replacement Cost

Generally speaking, you can expect to pay between $200-$450 to have your vehicle’s faulty O2 sensor replaced. The vast majority of this cost can be attributed to overall labor associated with completing replacement, while parts costs are typically far more mild.

This also poses a significant opportunity for the avid DIYer to save money, as most replacement O2 sensors can be purchased for $25-$100.

Can I Continue Driving With a Bad Oxygen Sensor?

A vehicle can technically continue to operate with a faulty oxygen sensor, though attempting to operate a vehicle under these circumstances for a prolonged period of time is not advised. Feedback from a vehicle’s O2 sensors are used to calculate an engine’s fuel trims, thus presenting the potential of over-fueling.

There are inherent risks that come with operating an engine for prolonged periods of time in an over-fueled state. The first of which surrounds potential damage to a vehicle’s catalytic converter, due to potential fuel contamination. This contamination causes overheating of the internal medium of this catalyst, leading to premature failure.

It’s also worth noting that attempting to operate a vehicle in an over-fueled state will lead to excessive fuel consumption, and resulting pain at the pump. Additionally, over-fueling can present problems when attempting to pass state or other municipality-mandated emissions testing.

Can an O2 Sensor Be Cleaned (Instead of Replaced)?

Attempting to clean a vehicle’s fouled O2 sensor is not recommended, as the chance of a favorable outcome is slim. An oxygen sensor’s element is more delicate than you might expect, making a sensor of this type far more difficult to salvage under circumstances when cleaning is required. 

You’d be far better off simply replacing a defective oxygen sensor, than attempting to clean it. The same amount of labor is required to complete either process, with far better chances of a successful outcome when actual replacement takes place.

Additionally, the cost associated with purchasing a replacement O2 sensor is rather nominal, in comparison to that of purchasing many other components.

What Causes an Oxygen Sensor to Fail?

Oxygen sensors can go bad for a number of reasons, though the most common is related to standard wear and tear. Sensors of this time are exposed to continuous heat, leading to loss of electrical integrity with time. Additionally, constant vibration should also be factored in, when considering what factors into normal wear and tear.

Aside from normal degradation, oxygen sensors are also prone to premature failure at the hands of contamination. It is not uncommon for O2 sensors to become exposed to raw fuel, in the event of prolonged rich conditions within one or more engine banks, or coolant, in the case of head gasket failure. Either circumstance can lead to untimely O2 sensor failure.

Diagnosing a Faulty O2 Sensor

The following steps can be used to assist in diagnosing a faulty oxygen sensor. As always, it is advisable to consult factory-specific service literature for your particular model of vehicle before attempting any such repairs.

#1 – Check for Additional DTCs

With the use of a quality scan tool, check for the presence of additional DTCs. Additional guidance can be gleaned from any O2-related DTCs that are present, possibly indicating which bank the issue at-hand corresponds to.

#2 – Inspect the Affected O2 Sensor

Begin by carefully inspecting the O2 sensor in question, as determined by the corresponding fault code that was logged. If this sensor appears damaged, replacement will be necessitated. Likewise, any wiring related damage within this circuit that is discovered, should be repaired before proceeding.

#3 – Test Affected O2 Sensor With Multimeter

Next, you will need to test the affected O2 sensor with a quality multimeter. This is best accomplished by back-probing the sensor’s signal wire with the meter’s positive lead while fixing the meter’s negative lead to sufficient ground.

With the engine running and up to temperature, upstream O2 sensor readings should constantly change, thereby reflecting the response. On the other hand, downstream O2 sensors should level out at an approximate reading within the mV scale.

Readings outside of this standard will require further diagnosis according to manufacturer standards, and possible O2 sensor replacement.

• Author
• Recent Posts

Josh Boyd

Josh has worked as a full-time mechanic for over 12 years, with 6 ASE (Automotive Service Excellence) certifications under his belt. He began his career as an automotive technician at local Toyota dealership and has since transitioned to specializing in diesel trucks and equipment.

Latest posts by Josh Boyd (see all)

• 6 Symptoms of a Bad Turbocharger (and Replacement Cost) - Jan 6, 2025
• 5 Symptoms of a Dirty Cabin Air Filter (and Replacement Cost) - Dec 30, 2024
• 5 Symptoms of a Bad Fan Clutch (and Replacement Cost) - Dec 12, 2024