O2 Downstream and Upstream Sensors: Can You Drive Without It? [Explained]

A car’s exhaust system is designed to meet emissions requirements by burning fuel more efficiently. Sensors are used to measure the car’s emissions.

The downstream and upstream o2 sensors, for example, tell your vehicle when it needs a tune-up or new catalytic converter so that it can be compliant with EPA guidelines. In this post I will explore the downstream and upstream o2 Sensors, and whether you can drive without them.

Table of Contents

What is upstream o2 sensor?

The functionality of the upstream o2 sensor is to monitor the level of pollutants in the engine’s exhaust and pass it to the ECU for continual adjustment of the air-fuel ratio. The upstream oxygen sensor is placed before the catalytic converter

The purpose is to maintain consistent levels of fuel efficiency and cut emissions. Catalytic converters help reduce toxic fumes from engines, converting harmful gases into less dangerous ones. Without sensors to tell the engine’s computer when the level of pollutants changes, the catalytic converter would require modification or replacement to remove different levels of toxins from the exhaust fumes.

The upstream oxygen sensor is designed to work with an ECU that adjusts for consistent air-fuel ratios. These are typically found on newer vehicles that fall into EURO 4 and EURO 5 emission standards.

What is downstream o2 sensor?

The downstream sensor examines the amount of pollutants flowing through the catalytic converter. The ECU uses data from the upstream and downstream sensors to compare them. When the readings from the two sensors become similar, the ECU triggers a catalytic converter error.

When the air/fuel mixture is at the stoichiometric ratio (14.7 parts air to 1 part fuel), the oxygen sensor produces 0.45 volts.

Can you drive without a downstream o2 sensor?

Yes, you can drive a car without a downstream o2 sensor. What happens is, if your downstream o2 sensor goes bad and you drive around, then the computer realizes that it’s not reading correctly and throws up a check engine light.

This means that it can no longer monitor for proper emissions after this point and your engine will run a rich fuel mixture, so fuel economy will drop.

Can you drive without an upstream o2 sensor?

Yes, you can drive for a short distance without an upstream o2 sensor. However, for a long drive, the ECU would detect there is no fuel in the exhaust and hence would inject more fuel to compensate. This will lead to clogged plugs and ultimately a gummy combustion chamber.

Can an O2 sensor be cleaned?

Basically, you can clean your o2 sensor, however, I do not recommend doing that because this sensor is sensitive and could be in a risk of being damaged which eventually alters the air-fuel mixture or worse, causing the ECU to trigger an engine alert.

How do I know if my upstream or downstream oxygen sensor is bad?

There are several signs that you might notice when your o2 sensor malfunctions:

Reduced gas mileage
Reduction in overall power
A gas smell coming from the exhaust

Check Engine Light
Rough idle
Misfire

Hesitation when trying to accelerate
Emission test fails

Can you bypass o2 sensors?

The answer is yes, you can bypass the oxygen o2 sensor by using a dummy (or fake ) O2 sensor. The use of a dummy o2 sensor is the only way to bypass an active faulty oxygen sensor and still pass emissions tests. A dummy o2 sensor will send to the ECU the correct parameters as a real good o2 sensor would.

Should I replace all my o2 sensors at once?

You should not replace all o2 sensors at once just because one sensor went bad. o2 sensors should be replaced as they fail, or as recommended by the car’s manufacturer.

How do I bypass the downstream o2 sensor?

Here are the steps in order to bypass downstream o2 sensor:

Car cool down – Before starting working on your car, I would suggest letting your car cool down for a couple of hours, as you are going to work on the engine and the exhaust systems

battery – Disconnect the negative battery terminal, and slide it aside.
Stands – Get stands to hold your car up. Place the stands under the front of your car. The best place to place them is on the back of your radiator. You will need to locate the pinch welds on your car, then you can put the jack under them.
Locate the o2 sensors– There are usually 2 in the back and 2 in the front of the catalytic converter.

o2 sensor wiring – Remove the electrical wire from the sensor carefully. Squeeze to release and then remove it gently from the housing of the oxygen/o2 sensor.
Remove the sensor – Rotate the o2 sensor counterclockwise.
Dummy o2 sensor – Plug in the dummy o2 sensor by rotating it clockwise, and re-connect the electrical wiring.

Check engine light – Once your car is on the ground, start the engine. Let the engine run on idle few minutes and verify the check engine light is off.

What is the difference between bank 1 and bank 2 on the o2 sensor?

The differences between bank1 and bank 2 are:

Bank 1 and bank 2 refer to either side of the engine.

Bank 1 is the one where cylinder #1 is located, and Bank 2 is the side with cylinder 2.
Bank 2 is the passenger side.
Bank 2 is post catalytic converter.

Bank 1 Sensor #1	Driver side before converter (front)
Bank 1 Sensor #2	Driver side after converter (rear)
Bank 2 Sensor #1	Passenger side before converter (front)
Bank 2 Sensor #2	Passenger side after converter (rear)

Differences between bank1 and bank 2 o2 sensors

Should I disconnect the battery when changing o2 sensor?

Yes, it is recommended to disconnect the battery before replacing the o2 sensor and wait for about 10 minutes before start working. Battery draining while disconnected will not take place because of ECU (Engine Control Unit) memory power loss.

Oxygen sensor signal information stored in ECU memory will be lost if you keep the battery connected while working on any electrical part under the hood.

Disconnecting the battery should be done every time you are working on any electrical problem related to your engine.

How long does it take for o2 sensor to reset?

It should take about 5-10 minutes for the o2 sensor to reset.

Turn the ignition key to “ON”, DO NOT start the engine. after no more than 10 minutes, the check engine should blink and then turn off. This means the o2 sensor has reset and the ECU has been restored. Now you can drive your car safely.

Can o2 sensor cause rough idle?

Yes, your o2 sensor can make your engine idle roughly. This is because the o2 sensor determines the air to fuel ratio. If the sensor goes bad, this function could be disrupted and cause a rough idle.