Upstream Vs Downstream O2 Sensor: What is the Difference?

Your vehicle’s downstream and upstream O2 sensors can tell you when it requires a tune-up or a new catalytic converter in order for it to be in conformity with the regulations set out by the EPA. In this article, we will discuss the Upstream Vs Downstream O2 Sensor, whether or not it is possible to operate an automobile without them, as well as “how do I know if I need an upstream or downstream oxygen sensor”?

What is the Upstream O2 Sensor?

The upstream oxygen sensor’s job is to keep an eye on the quantity of pollutants in the air-fuel mixture and relay that information to the engine control unit (ECU), which makes constant adjustments to the air-to-fuel ratio. This sensor can be found placed before your car’s catalytic converter.

The goal is to achieve and sustain high levels of fuel economy while simultaneously reducing emissions. Catalytic converters enable engines to produce less polluting exhaust emissions by transforming harmful gasses into less hazardous byproducts. But they would have to be replaced in the absence of upstream and downstream sensors responsible for communicating fluctuating quantity of pollutants in the air fuel mixture to the ECU. 

Speaking of ECU, it is an electronic control unit (ECU) that maintains a constant air-to-fuel ratio and functions in tandem with an upstream oxygen sensor. Both these sensors are commonly seen on modern automobiles that meet the emission standards EURO 4 and EURO 5.

What is a Downstream O2 Sensor?

The role of a downstream o2 sensor is to measure how effective is the catalytic converter with the removal of toxic pollutants, Carbon monoxide, hydrocarbons, and nitrogen oxides, to name a few. Both the sensors send the collected data to ECU, which then compares the readings against standardized regulations, and if the readings become identical, the electronic control unit will signal a problem with the catalytic converter.

Is it Possible to Drive a Car That Doesn’t Have a Downstream O2 Sensor?

It is possible to operate a vehicle that does not have a downstream oxygen sensor. If your downstream oxygen sensor fails and you continue to drive the vehicle, the onboard computer will eventually understand that the data it is receiving is not accurate and will activate the “check engine” warning light.

Your engine will run rich in fuel, which will decrease the fuel efficiency and ultimately the mileage.

Is it Possible to Drive a Car That Doesn’t Have an Upstream O2 Sensor?

Yes, it is possible to drive your car for a short distance without an upstream oxygen sensor. On the other hand, the ECU will notice that there is no longer any gasoline in the exhaust after a lengthy drive and will thus inject additional fuel to make up for it. Because of this, the plugs will get blocked, and eventually, the combustion chamber will become sticky.

Can You Clean Your Car’s O2 Sensors?

Yes, you can clean your car’s o2 sensor, however, we do not recommend that you do so because the sensors are sensitive and could be at risk of being damaged, which would eventually alter the fuel-air mixture or, worse, cause the ECU to initiate an engine alert. If you insist on cleaning your o2 sensor, keep in mind that you are doing so at your own risk.

How to Identify If an O2 Sensor is Bad?

upstream vs downstream
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If your O2 sensor is malfunctioning, you may observe a number of warning indications, including the following:

  • A gas smell that is coming from the exhaust
  • Check Engine Light
  • Emission test fails
  • Hesitation when trying to accelerate
  • Misfire
  • Reduced gas mileage
  • Reduction in overall power
  • Rough idle

Can O2 Sensors Be Bypassed?

The answer is Yes!  You are able to get around the oxygen O2 sensor if you use a dummy, sometimes known as a false, O2 sensor. The only method to get around a working oxygen sensor that is malfunctioning while still passing emissions testing is to make use of a “dummy” oxygen sensor. A fake oxygen sensor will provide the same parameters to the engine control unit (ECU) as a genuine oxygen sensor would.

Should We Replace All the O2 Sensors?

Just because one of the oxygen sensors failed does not mean that you should immediately replace the others. O2 sensors should be changed as soon as they become defective or as the manufacturer of the vehicle recommends.

How to Bypass a Downstream O2 Sensor?

In order to avoid the downstream oxygen sensor, the following procedures need to be taken:

  1. Let the Car Be Cooled Down

As you are going to be working on the engine as well as the exhaust systems, we would advise you to let your vehicle cool down for a few hours before you start working on it, as this will make your job much safer.

  1. Disconnect the Battery

Pull the negative battery terminal away from its connection and set it to the side.

  1. Position the Stands

Position the stands so that they are below the front of your vehicle. The rear of your radiator is the ideal location for putting them in your home. You will first need to find the pinch welds on your vehicle, and after you have done so, you will be able to position the jack below them.

  1. Locate the o2 Sensors

These are located in the rear and in the front of your car’s catalytic converter.

  1. O2 Sensor Wiring

Carefully remove the connection between the electrical cable and the sensor. After you have released it by squeezing it, take care to remove it from the housing of the O2 sensor.

  1. Remove the sensor

Turn the oxygen sensor so that it is facing anticlockwise.

  1. Dummy o2 sensor

Reconnect the electrical wire and make sure the dummy oxygen sensor is plugged in by turning it in the clockwise direction.

  1. Check engine light

Start the engine as soon as your vehicle is safely on the ground. Check that the “check engine” light has been turned off after letting the engine run at idle for a few minutes.

Should the Battery Be Disconnected When Changing the O2 Sensor?

It is advised that the battery be disconnected before changing the O2 sensor, and then you should wait for about ten minutes before you begin working. As a result of the lack of electricity to the ECU’s (Engine Control Unit’s) memory, the battery will not discharge while the device is disconnected.

If you keep the battery attached while working on any electrical component under the hood, the information about the oxygen sensor signal that has been saved in the ECU memory will be lost.

When dealing on any electrical issue that is linked to your engine, you should always disconnect the battery first.

How Much Time Does an O2 Sensor Take to Reset?

The resetting of the oxygen sensor ought to take between five and ten minutes.

While you turn the ignition key to the “ON” position, you should NOT start the engine. The check engine light should start blinking after no more than ten minutes and then turn off after that. This indicates that the O2 sensor has been reset. Now you may drive your automobile securely.

Can O2 Sensor Result in a Rough Idle?

There is some truth to the assertion that the O2 sensor might cause your engine to idle roughly. This is due to the fact that the o2 sensor is responsible for determining the air-to-fuel ratio. It is possible that if the sensor fails, this function may be disturbed, which will result in a rough idle.

The Bottom Line

So that is all we had to tell you in this downstream vs upstream o2 sensor guide. Oxygen sensors are used inside the exhaust system in order to measure the levels of pollutants and impurities that are expelled with the exhaust.

The placement of the O2 sensor is used to determine whether it is upstream or downstream; an upstream sensor is one that is placed either before or after the catalytic converter.

It is possible for O2 sensors to malfunction, which will result in the illumination of the “check engine” warning light as well as a number of other peculiar symptoms.

If you have any queries regarding Upstream Vs Downstream O2 Sensor, please feel free to write to the Car Expert Group and we will be happy to assist you.

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