What Is an Engine O2 Sensor?

By | November 1, 2022

What Is an Engine O2 Sensor
The O2 or oxygen sensor is a major component in modern fuel injection engines and allows for better efficiency and emissions.

It is the main sensor the onboard computer (ECM) receives data from when regulating the fuel-air ratio mixture.

The O2 sensor affects everything from fuel economy to performance and catalytic converter efficiency.

How Does the O2 Sensor Work?

How Does the O2 Sensor Work
The O2 sensor works in combination with the onboard computer and fuel injectors to get the best fuel-air mixture ratio possible.

The O2 sensors are located on the exhaust so they can determine how much oxygen is left after the cylinders burn a mixture of air and fuel.

If there is High Oxygen, this means there is a lean condition with too much air.
What Is the O2 Sensor Lean

If there is High Oxygen, this means there is a rich condition with too much gas.

The 02 sensor produces a voltage based on the difference between the oxygen inside the exhausts as compared to the oxygen outside the exhaust.

The voltage produced falls between zero and one volt, with the ideal fuel mixture at nearly half a volt.

If there is a lot of oxygen in the exhaust, the sensor produces little voltage, which can indicate a lean condition.
Lean Condition

If there is very little oxygen in the exhaust, the sensor produces a higher voltage indicating a possible rich condition.

The vehicle’s computer receives these signals and adjusts the fuel mixture to keep the mixture as close to perfect combustion as it can get.

The computer adjusts the fuel mixture by changing how long the fuel injectors are open, adding or subtracting fuel to maintain the best fuel-air mixture.

How To Troubleshoot a Bad O2 Sensor?

An OBDII scan tool will be needed to read the codes stored in the onboard computer (ECM).

There are many low-cost scan tools that will give an oxygen sensor code.

Most Auto-part stores will also usually do a scan for free if you do not have one.

Once the code is known, the bad sensor can be identified and replaced.

OBDII Error Codes for O2 Sensor

  • P0130 O2 Sensor Circuit (Bank 1, Sensor 1)
  • P0134 O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 1)
  • P0138 O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
  • P0150 O2 Sensor Circuit Malfunction Bank 2 Sensor 1
  • P0151 O2 Sensor Circuit Low Voltage Bank 2 Sensor 1
  • P0152 O2 Sensor Circuit High Voltage Bank 2 Sensor 1
  • P0153 O2 Sensor Circuit Slow Response Bank 2 Sensor 1
  • P0154 O2 Sensor Circuit No Activity Detected Bank 2 Sensor 1
  • P0155 O2 Sensor Heater Circuit Malfunction Bank 2 Sensor 1
  • P0156 O2 Sensor Circuit Sensor Malfunction Bank 2 Sensor 2
  • P0157 O2 Sensor Circuit Low Voltage Bank 2 Sensor 2
  • P0158 O2 Sensor Circuit High Voltage Bank 2 Sensor 2
  • P0159 O2 Sensor Circuit Slow Response Bank 2 Sensor 2
  • P0160 O2 Sensor Circuit No Activity Detected Bank 2 Sensor 2
  • P0161 O2 Sensor Heater Circuit Malfunction Bank 2 Sensor 2
  • P0162 O2 Sensor Circuit Malfunction Bank 2 Sensor 3
  • P0163 O2 Sensor Circuit Low Voltage Bank 2 Sensor 3
  • P0164 O2 Sensor Circuit High Voltage Bank 2 Sensor 3
  • P0165 O2 Sensor Circuit Slow Response Bank 2 Sensor 3
  • P0166 O2 Sensor Circuit No Activity Detected Bank 2 Sensor 3
  • P0167 O2 Sensor Heater Circuit Malfunction Bank 2 Sensor 3
  • P0170 Fuel Trim Malfunction Bank 1
  • P0171 System Too Lean Bank 1
  • P0172 System Too Rich Bank 1
  • P0173 Fuel Trim Malfunction Bank 2
  • P0174 System Too Lean Bank 2
  • P0175 System Too Rich Bank 2

How To Locate the O2 Sensor?

An OBDII scan tool will give a code along with the Bank and Sensor number.

For example, the P0031 Error Code is a Bank 1 Sensor 1.

Bank 1 is always the side of the engine with the number 1-cylinder.

A V6 or V8 engine will have two-banks, while a 4-cylinder engine will only have one bank.

Oxygen Sensors are on the exhaust both before and after the catalytic converter.

The one before the catalytic converter is called sensor-1 or upstream.

The one after the catalytic converter is called sensor-2 or downstream.

What Happens to an Engine When the O2 Sensor Goes Bad?

  • Rough Idle
  • Surge
  • Hesitation
  • Misfire
  • Lack of Power

When an oxygen sensor goes bad, the engine light will usually come on, and the engine may idle roughly.

How badly the engine runs will depend on how badly the 02 sensors have failed.

If it has failed completely, the vehicle will likely be undrivable and need to be fixed immediately.

If the sensor is only failing occasionally, the vehicle may still run well, but the engine light comes on.

Once the sensor has begun to fail, it is best to get it replaced as it will only get worse.

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