Oxygen Sensor-Should you Replace It ?


Replacement of good oxygen sensors is one of the common mistakes made by seasoned mechanics and DIY mechanics when tackling rich or lean diagnostic codes.


Oxygen Sensor-Should you Replace It ?

Before replacing any of these components, you must first find the real problem that caused the failure. The purpose of this post is to inform you, in the strongest possible way, of the consequences of not diagnosing and checking the systems that make an engine operate.

It is important to remember that today’s vehicle is completely controlled by computers and sensors.

There may be as many as 20 sensors feeding information to the vehicle “master control unit “ (MCU) or brain box.

There are other sensors, while not directly attached to the MCU, that do affect the information the sensors feed to computers.

These sensors are part of systems that operate the vehicle properly.

All of the systems must be in proper operating order for the engine to function properly and have normal combustion.

What should we check when we have those rich or lean codes?

The most common issues for lean codes are:

  • Vacuum leaks, check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
  • Restricted fuel filter or bent/pinched fuel system lines
  • Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
  • Engine misfire, Yes I know this one may seem weird. You might think that if there is a misfire then you will have all that unburned fuel and it should read rich; right? Well the O2 sensors read only oxygen content in the exhaust, so if you have all that unburned fuel from incomplete combustion then, you guessed it, you also have all that unburned oxygen. High O2 content in exhaust equals a lean reading!

There are also some other possibilities such as an internally leaking EGR system,.

This will typically set a separate code. A leak in the exhaust system before the O2 sensor will also cause incorrect readings.

And always check for after- market modifications. These can throw a wrench into the works!

The only other possibilities (however unlikely), are wiring issues, computer concerns or a bad O2 sensor.

The possible causes of rich codes are:

1. A leaking or faulty fuel injector
2. Fuel injector driver in computer shorted, or wiring short for injectors (likely a ground short)
3. Leaking or faulty fuel pressure regulator or restricted return line
4. Faulty evaporative emissions system – bleeding fuel vapors into engine (not commanded by computer)
5. On newer models a faulty fuel pump or fuel pump driver module
6. Faulty readings from other sensors such as a Mass Air Flow Sensor. You may actually be getting more air than the MAF tells the computer
7. Exhaust leaks before the sensor will cause erratic readings
8. After market components or performance chips
9. And yes, if I dare say it, possibly a computer, wiring issue or even a faulty O2 sensor!



The other codes we should address are those related to the sensors located after the catalytic converter. Though these may appear identical to the oxygen sensors pre-converter, they perform an entirely different task and are known as Monitors. The only job of these sensors is to “monitor” the efficiency of the catalytic converters.

The readings from these sensors should be much more stable and not fluctuating like the front O2 sensors. The computer compares the readings from the oxygen sensors (pre cats) and the monitors (post cat) to determine if the catalytic converters are doing their job and “cleaning” the exhaust. You never want to replace a monitor for a rich/lean concern as they have no bearing on these codes.

As the converters begin to fail, you will see the monitors voltage readings follow the oxygen sensor readings. Technically these are all “oxygen sensors” but it is important to distinguish the difference between pre-converter & post converter sensors, so I find it easiest to stick to calling the back ones monitors.

What To Look For
What To Look For

Why an Oxygen Sensor May Go Bad

The oxygen sensor in modern cars can last up to 100K miles, but typically you would experience problems sooner than that. Over time, an oxygen sensor may become caked with byproducts of combustion, such as sulfur, lead, fuel additives, oil ash, etc. This contamination causes the sensor to lose its ability to produce voltage and send the right signal.

Using fuel that is not recommended for your vehicle or using low-quality gasoline may also speed up the oxygen sensor failure. And if you are skipping maintenance, especially things like timely spark plug and air filter replacement, you are increasing the likelihood of incomplete fuel combustion, which in turn leads to more dirt and grime in your emissions system.

Oxygen Sensor
Oxygen Sensor

Signs of a Bad Oxygen Sensor

In most cases, a bad oxygen sensor will trigger a check engine light. P0138 and P0135 are some of the codes you may expect to see on the OBD II reader if you have one. Other than that, it’s difficult to spot a failing oxygen sensor. It will inevitably lead to decreased gas mileage, but it’s usually not drastic enough for an average driver to notice. The decrease is gradual and happens over time, so unless you are keeping tabs on your MPGs, you will likely miss these signs. A bad or failing O2 sensor can also cause you to fail your emissions test.

When to replace the sensor

If your vehicle has been manufactured within the past 15 years, your oxygen sensor should be replaced every 60,000 to 90,000 miles. The oxygen sensor is a wear item and does fail over time. This will reduce the level of emissions your vehicle puts into the atmosphere, while keeping your engine running smoothly and properly.

If you have a Check Engine light, poor gas mileage, or an irregular engine idle, it could be time to change it !


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