Oxygen Sensor (O2) – What Else Could Cause Rich Or Lean Codes

Oxygen Sensor (O2) - What Else Could Cause Rich Or Lean Codes
Oxygen Sensor (O2) - What Else Could Cause Rich Or Lean Codes

Replacement of a good Oxygen Sensor (O2), is a common mistake made by (DIY) mechanics all the time.

So, before replacing any of these sensors; you must first find the real problem that pushed you in that direction.

The consequences of not diagnosing and checking the systems that make an engine operate properly can be costly.

The Oxygen Sensor also known as just (O2) sensor, is a wear item and does fail over time.

So, it is important to remember that today’s vehicles, are 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. Consequently, these sensors are part of other systems, that operate together to make the engine run efficiently.

What Should We Check When We Have Those Rich Or Lean Codes
What Should We Check When We Have Those Rich Or Lean Codes

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

What Should We Check, When We Have Those Rich Or Lean Codes ?

Basic Oxygen (O2) Sensor Codes
Basic Oxygen (O2) Sensor Codes

If there is too much fuel and not enough air; the engine is said to be “running rich”, or “has a rich mixture”. It will have a gassy or rotten egg smell from the exhaust; give off a burning effect to the eyes and will make black smoke.

If there is too much air and not enough fuel; the engine is said to be “running lean.” or “has a lean mixture”.

The most common issues for lean codes are:

Vacuum Leaks

  • Check for failed or loose vacuum lines, leaking intake gaskets, or any other source of unmetered air leaks.

Clogged Filter Or Lines

  • Restricted fuel filter or pinched fuel lines.

Faulty Sensors

Engine Misfires

  • The O2 sensors only read (O2) content in the exhaust. So, if you have all that unburned fuel from incomplete combustion then; you also have all that unburned (O2). High (O2) content in exhaust equals a lean reading, also causing engine misfires.
Used (O2) Sensor Comparison
Used (O2) Sensor Comparison

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. The only other possibilities are wiring issues, and computer concerns.

The Possible Causes Of, Rich Codes Are:

  • A leaking or faulty fuel injector
  • Fuel injector driver in computer shorted, or wiring short for injectors (likely a ground short)
  • Leaking or faulty fuel pressure regulator or restricted return line
  • Faulty evaporative emissions system – bleeding fuel vapors into engine (not commanded by computer)
  • On newer models a faulty fuel pump or fuel pump driver module
  • 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.
  • Exhaust leaks, before the sensor will cause erratic readings

Sensor Monitors

The other codes we should address, are those related to the sensors located after the catalytic converter. Though these may appear identical to the (O2) Sensor 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 (O2) sensor (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 (O2) sensor readings. Technically these are all “Oxygen Sensor” 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.

Why, An Oxygen Sensor May Go Bad

Bad (O2) Sensor
Bad (O2) Sensor

The (O2) sensor can last up to 100K miles; but typically you would experience problems sooner than that. Over time, an (O2) 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.

This is critical for maintaining low emissions and good fuel economy. If an (O2) sensor gets “lazy” because of old age or contamination; the computer may not be able to adjust the fuel mixture quickly enough, as the engine’s operating conditions change. (O2) sensors that are failing tend to read lean; which causes the fuel system to run overly rich to compensate.

Signs Of A Bad Oxygen Sensor

(O2) Comparison Illustration
(O2) Comparison Illustration

In most cases, a bad (O2) sensor will trigger a check engine light. P0138 and P0135 are some of the codes you may expect to see on a OBD II reader. Other than that, it’s difficult to spot a failing (O2) sensor. It will inevitably lead to decreased gas mileage; but it’s usually not drastic enough for an average driver to notice. Also, a bad or failing O2 sensor, can also cause you to fail your emissions test.

When To Replace The Oxygen (O2) Sensor

Always follow the instructions in your owner’s manual, for Oxygen (O2) Sensor replacement. The Oxygen (O2) Sensor is a wear item and does fail over time. This will reduce the level of emissions your vehicle puts into the atmosphere. And, at the same time, keep your engine running smoothly.


So, what happens to Oxygen (O2) Sensors is, that they tend to become fouled with carbon and sooty deposits. The element just simply erodes and wears down like the electrode on a spark plug.

Finally, neglecting to replace a bad Oxygen (O2) Sensor, will usually result in damage to your catalytic converter.

Thank You !