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Common Symptoms of Faulty Oxygen Sensor(s) (O2S)

By R&D
Published on August 19th, 2024

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The Oxygen Sensor (O2S) is a critical sensor for proper engine operation. O2Ss are located both before (upstream) and after (downstream) the catalytic converter. Oxygen Sensors measure oxygen concentration in the exhaust stream, reporting O2 levels to the Engine Control Module (ECM) by voltage signals (or current signals for wideband O2S).

The downstream O2S monitors the catalytic converter to ensure it is operating properly. The upstream O2S measures the fluctuating concentration of oxygen in the exhaust gas playing a crucial role in adjusting the amount of fuel in the air-fuel mixture. If it malfunctions, it can have significant consequences for vehicle performance, emissions, and overall reliability. This article highlights the common symptoms of a faulty upstream O2S, providing insight into potential problems and their underlying causes.

What Causes Oxygen Sensor Failure?

Physical Damage: Physical damage to oxygen sensors can occur due to impact, vibration, exposure to extreme conditions, improper installation, or corrosion. These conditions can affect the performance and lifespan of O2S.

Contamination: Contaminants such as oil, coolant, silicone, or fuel additives can degrade the sensing element of the oxygen sensor over time. These substances can coat the sensor’s sensing element, inhibiting its ability to detect oxygen levels accurately.

Age and Wear: As with many automotive components, oxygen sensors have a finite lifespan and degrade over time from exposure to normal engine operation. The sensing element may become less responsive, leading to reduced performance and increased emissions.

Electrical Malfunction: Faulty wiring, connectors, or electrical shorts in the O2S circuit can prevent the sensor from sending accurate signals to ECM, leading to performance issues or false readings.

Common Symptoms of a Faulty O2S

Malfunction Indicator Light (MIL) or Check Engine Light (CEL) Illuminates

A malfunctioning O2S may trigger the vehicle’s onboard diagnostics system to illuminate the MIL and store a corresponding Diagnostic Trouble Code (DTC) in its memory. An OBD2 scanner can quickly identify which DTC has commanded the MIL on.

Black Exhaust Fumes

When the O2S sends an incorrect signal to ECM, such as when the engine is operating normally or in a rich condition, but the O2S sends a “low” voltage to the ECM (indicating a “lean” condition), the ECM commands injecting more fuel, leading to incomplete combustion and black smoke from the vehicle’s tailpipe. Similarly, when the engine is running in a “rich” condition, and the O2S does not send an appropriate signal to ECM, the result is also incomplete combustion and black smoke from the tailpipe. This adversely affects combustion efficiency, leading to unburned fuel, backfiring, or soot-like smoke emitted from the vehicle’s exhaust – as well as high fuel consumption, poor idling, and hard-starting issues. These conditions may cause the vehicle to fail emissions tests, and contribute to environmental pollution.

Increased Fuel Consumption

An O2S that is sluggish or has failed altogether can send incorrect readings to the ECM, resulting in an overly rich air-fuel mixture and increased fuel consumption.

Unstable Idling

A faulty O2S will affect proper engine operation. When the oxygen sensor is faulty, it may provide inaccurate readings to the ECM, causing it to incorrectly adjust the air-fuel ratio. This can result in a mixture that is either too rich (excess fuel) or too lean (excess air). These conditions can result in rough or irregular engine idling, rough running, or even stalling, particularly when the vehicle is at idle or during low-speed operation.

Poor Performance While Driving

A malfunctioning O2S may cause the engine to hesitate or exhibit poor acceleration, especially during sudden throttle changes or when accelerating from a stop. This can result in sluggish performance and reduced responsiveness.

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Identify A Faulty O2S with the Innova Scan Tool

The Innova Scan Tool is a highly useful diagnostic device designed to quickly identify problems within a vehicle's systems. It is particularly effective in diagnosing and resolving issues related to O2S by detecting DTCs associated with it. In simpler terms, this tool helps efficiently diagnose O2S-related problems in vehicles, helping mechanics and car owners pinpoint and address issues with ease.

In addition, you can also read the individual Oxygen Sensor values with the Innova Scan Tool to determine if the O2S value is low or not. The normal operating value of the O2S depends on your vehicle.

The O2S symbol depends on the manufacturer but is usually:

Bank 1 is the side with the #1 cylinder (Cylinders 1 – 3 – 5 – 7).

Bank 2 is the side with the #2 cylinder (Cylinders 2 – 4 – 6 – 8).

Sensor 1 is located before Catalytic Converter Front (Upstream O2S).

Sensor 2 is located after Catalytic Converter Rear (Downstream O2S).

The common voltage range of the O2S is approximately 0.1V – 0.9V, where 0.1V – 0.2V indicates a “lean” mixture, 0.45 – 0.55V indicates the stoichiometric (ideal) air-fuel mixture, and 0.8V – 0.9V indicates a “rich” mixture. Note that these values may vary between manufacturers.