How To Tell If Upstream or Downstream O2 Sensor Is Bad

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Whether it is the upstream O2 sensor or the downstream O2 sensor that went bad, the signs and symptoms of a bad upstream or downstream O2 sensors are the same; and that includes check engine light illumination, loud noise from the engine, and serious shaking while driving.

Just like every other component that makes up a car, the oxygen sensors can be physically damaged. When the O2 sensors go bad, you will start noticing several unusual signs.

It is then expected that you act proactively to replace the faulty sensor(s) before the issue starts affecting other components that are connected to O2 sensors.

In this article, you will learn how to tell if the upstream or downstream O2 sensor is bad and what to do about it.

Let’s get started!

How To Tell If Upstream or Downstream O2 Sensor Is Bad

First things first, how do upstream and downstream O2 sensors work? A better understanding of how they work would help us to arrive at the perfect answer for this question.

Upstream sensors are installed before the catalytic converter and what they do is monitor the level of unburnt oxygen (pollutants) in the emissions sent from the engine’s combustion chamber.

This information monitored by the upstream sensors is then sent to the ECU where it is processed and used for regular fuel/air delivery.

Similarly, downstream O2 sensors monitor the level of unburnt oxygen (pollutant) in the emissions released by the catalytic converter. Also, the information recorded by downstream sensors is sent to the ECU for further processing.

The major difference between upstream and downstream oxygen sensors is that the former (upstream) is installed before the catalytic converter, while the latter (downstream) is installed behind/after the CAT, just before the muffler or resomer.

Since upstream and downstream oxygen sensors perform the same task – monitoring the level of pollutants in emissions – they exhibit the same signs and symptoms when any of them goes bad.

So to say, whether it is the upstream O2 sensors that got bad or the downstream O2 sensors, the signs you’d get is the same.

Now, how would you know the exact sensor that got bad (since they exhibit the same signs)? This is where “troubleshooting” comes in to help.

To determine which of among the O2 sensors got bad, you need to use an OBII scanner to troubleshoot the sensors, or a voltmeter to read the voltage transmission of each sensor.

The process is quite technical and you may need a mechanic to help you with reading and interpreting the codes. Notwithstanding, having all your O2 sensors changed is not expensive and doesn’t take time.

What More?

The symptoms of bad O2 sensors (whether upstream or downstream) includes:

Increased fuel usage
Poor performing engine
Your car won’t pass emissions tests
Unusual engine misfires
Check engine light is on

So, how to tell if upstream or downstream O2 sensor is bad? When you see the signs, mentioned above, it’s clear indications that you need to check on your O2 sensors, and probably have them replaced.

Lastly, while upstream and downstream O2 sensors perform similar functions, they don’t have the same physical design and cannot be used interchangeably.