HomeTips & GuidesHow To Check O2 Sensor: Testing the Upstream and Downstream Oxygen Sensors

How To Check O2 Sensor: Testing the Upstream and Downstream Oxygen Sensors

We will look at how to check O2 sensors when something is amiss below. 

Most vehicles usually have two oxygen sensors installed along the exhaust pipe before and after the catalytic converter. 

These components ensure the engine maintains the correct air-fuel ratio to reduce emissions and fuel consumption. 

So you can expect reduced engine performance when they fail, and here’s how to test these devices.

Table of Contents

What Is an Oxygen Sensor?

Oxygen sensors are devices that measure the oxygen content in the exhaust gas coming from the combustion chambers in internal combustion engines. 

These motors require a precise air-fuel ratio to burn efficiently.

The engine burns a lean mixture if the oxygen level is too high. 

And if too low, the engine burns a rich mix. The sensor sends this data to the powertrain control module or ECU to make the necessary adjustments.

An upstream oxygen sensor plugged into the engine

An upstream oxygen sensor plugged into the engine

Why Oxygen Sensor Data Is Necessary

An engine running with too much air (lean fuel mixture) can jerk or hesitate when accelerating. 

On the other hand, when it runs using little air (rich fuel mixture), it can get hot and produce more harmful gasses.

Both cases can damage the engine, so it is vital to keep this sensor in check to prevent expensive repairs.

Signs of Faulty Oxygen Sensors

Vehicles can exhibit these signs if they have damaged oxygen sensors.

  • Check engine light is on
  • Poor fuel economy
  • Rough idling
  • Stalling
  • Poor acceleration/performance
  • Black smoke from the tailpipe due to running rich

How To Check O2 Sensors

Oxygen sensors have zero moving parts, meaning they have long lifespans. 

So failing is uncommon, but it does not mean they last forever.

 Issues like contaminants, electrical faults, high mileage (age), and poor engine condition can make them fail.

So it is better to test these components after a specific mileage.

 For instance, consider testing heated oxygen sensors after about 60,000 miles and their unheated counterparts after about 30,000 miles.

Oxygen Sensor Location

All vehicles built after 1996 have upstream and downstream oxygen sensors.

The upstream unit detects oxygen levels before the catalytic converter, then sends the oxygen level data to the PCM or ECU for air-fuel ratio adjustment. 

But the downstream sensor (after the converter) checks if the catalytic converter is functioning efficiently to convert the harmful exhaust fumes.

A downstream oxygen sensor

A downstream oxygen sensor

So cars with straight or inline engines have one exhaust manifold that directs the exhaust gasses to a single catalytic converter with two oxygen sensors. 

But those with V configurations, such as V6 and V8, have four oxygen sensors because each cylinder head has a separate manifold, meaning two catalytic converters (two sensors for each).

Therefore, you should check the upstream and downstream O2 sensors to confirm if everything is okay. And they should produce different results.

An exhaust manifold for direct exhaust gasses from four combustion chambers to a catalytic converter in the exhaust pipe

An exhaust manifold for direct exhaust gasses from four combustion chambers to a catalytic converter in the exhaust pipe

How To Check the Upstream O2 Sensor

We will use an OBD2 scanner for this test, and we recommend getting one with a data stream function to read the live sensor data when the engine is on.

Oxygen sensors start operating at a temperature of around 600ºF. So warm up the engine by revving it at 2,000 RPM for 15-20 seconds to get the device to peak operating temperature. 

When heated, the device will begin generating the voltage signal to send to the ECU.

Gas engines typically require 14 grams of oxygen to burn a single gram of fuel efficiently. 

Upstream oxygen sensors generate low voltages when the exhaust gas has high oxygen levels (lean exhaust gas). 

This condition makes the ECU increase the fuel ratio in the mix.

But the sensor generates a high voltage if the exhaust gas has low oxygen levels (rich fuel mixture). 

The ECU interprets this voltage as a signal to reduce fuel in the mix.

So this upstream O2 sensor maintains the proper air-fuel ratio by flip-flopping between lean and rich air-fuel mixtures. 

Therefore, the voltage readings in the scanner should constantly fluctuate.

An upstream oxygen sensor

An upstream oxygen sensor

The waveform should not exceed 0.9V on the upper level and 0.1V on the lower scale on each cycle. 

One cycle is complete when the signal crosses the 0.45V point three times. 0.45V is the bias voltage

So you can know the sensor is faulty if you encounter any of these readings.

Scenario 1: Reading Stuck at 0.45V

The sensor might have these issues if the wave fluctuates slightly around the bias voltage.

  • Wiring problems, such as shorting
  • Failure
  • Faulty ground connection

Oxygen sensors in some Jeep and Chrysler vehicle models can generate voltages around 2V or 4V when they have open circuits (discontinuity) or faulty sensors.

Scenario 2: High Voltage Fluctuation (Above 0.45V)

Remember, the oxygen sensor generates a high voltage when it detects low oxygen levels. 

So if it constantly generates waveforms above the bias voltage (0.45V-9V), it is not faulty. The issue is somewhere else in the engine.

And the most probable causes include the following:

A mass airflow sensor in the air intake system

The scope of these errors covers any fault that increases fuel delivery to the combustion chambers that exceeds the normal levels.

Scenario 3: Low Voltage Fluctuation (Below 0.45V)

The oxygen sensor generates low voltages when the exhaust gas has high oxygen levels (lean exhaust gas). 

So the voltage waveform will fluctuate between 0.1V and 0.45V. And in this case, the sensor is not faulty because it generates a voltage.

The culprit is usually a fault that decreases fuel flow or increases airflow to the engine, making the air content exceed the 14:1 ratio. Leaks are also common. 

These faults can be either of the following.

  • Throttle body vacuum leaks
  • Exhaust leaks
  • Dirty fuel injection or mass airflow sensor
  • Restricted fuel line
  • Weak/faulty fuel pump
A mechanic cleaning a vehicle’s throttle body

A mechanic cleaning a vehicle’s throttle body

So check and repair these parts, then monitor the readings to see if the fluctuation is back to normal.

How To Check the Downstream O2 Sensor

Unlike the upstream oxygen sensor, this downstream O2 sensor signal should have minimal fluctuations (almost a straight line) around 0.5V when functioning normally. 

So if you notice oscillations in the graph readings, the catalytic converter is not optimally handling the reduction and oxidation processes.

A catalytic converter with an oxygen sensor attached downstream

A catalytic converter with an oxygen sensor attached downstream

The cause for these fluctuations can be either of the following.

  • Faulty connector wires
  • Dirty mass airflow sensor
  • Vacuum leaks
  • Dirty air filter
  • Defective spark plugs

So check these probable faults before replacing the downstream O2 sensor.

How To Check O2 Sensor Using Error Codes

Testing and monitoring these upstream and downstream O2 sensor voltages is not as easy as it sounds. 

Luckily, there is an alternative method for checking these devices, which is by using fault codes.

This process also needs an OBD2 scanner. But it’s not necessary to have a data stream function; we won’t be reading a graph.

After connecting the scanner to the connector slot in your vehicle, turn on the ignition to enable communication with the ECU. 

Go to the main menu and select “diagnose,” then “read codes.”

An OBD2 scanner scanning for error codes from the engine’s computer

An OBD2 scanner scanning for error codes from the engine’s computer

The scanner will indicate the error codes, but you might get confused with the message, especially when dealing with V configuration engines.

 Since these have four oxygen sensors, you need to know the following.

(1) Bank 1, sensor 1 refers to the upstream O2 sensor on the side with odd number cylinders (1, 3, 5, 7, etc.)

(2) Bank 1, sensor 2 is the downstream O2 sensor on the side with odd cylinders (1, 3, 5, 7, etc.)

(3) Bank 2, sensor 1 refers to the upstream O2 sensor on the side with even cylinder numbers (2, 4, 6, 8, etc.)

(4) Bank 2, sensor 2 is the downstream O2 sensor on the side with even cylinder numbers (2, 4, 6, 8, etc.)

Common trouble codes include P0130, P0131, P0135, P0171, P0174, and P0175.

 Dig deeper to understand why the ECU throws these fault codes because it could be a secondary issue, not a faulty sensor.

Try using tools like OBD2 code lookup to find the meaning of the code for your specific vehicle make.

Wrap Up

Checking the O2 sensor voltage chart or engine code is the easy part. 

Finding the issue can take time because the scanner data usually points you in a general direction. So you can check other symptoms to narrow down the problem.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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