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Common Symptoms of a Faulty Air/Fuel Ratio Sensor (AFR)

By R&D
Published on November 4th, 2024

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The Air-Fuel Ratio (AFR) sensor, also called a wideband Oxygen Sensor (O2S), is a crucial component in modern internal combustion engine systems. It measures the oxygen content in the exhaust gas and provides feedback to the Engine Control Module (ECM) to optimize fuel injection.

The ECM uses this data to adjust the fuel injection, ensuring the air-fuel mixture remains at its optimal ratio. By continuously analyzing the oxygen content in the exhaust gas, the sensor allows the ECM to make real-time adjustments to the fuel delivery, enhancing engine performance and minimizing emissions. Therefore, if the AFR sensor fails, it causes significant issues for engine efficiency and results in increased emissions. This article highlights the common symptoms of a faulty AFR sensor, helping you to identify potential problems and their underlying causes.

Common Symptoms of a Faulty Air/Fuel Ratio Sensor (AFR)

What Causes AFR Sensor Failures?

Physical Damage: Impacts, vibrations, exposure to extreme conditions, improper installation, or corrosion can cause fractures and deteriorate the sensor material, affecting the performance and lifespan of the AFR sensor.

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

Age and Wear: The AFR sensor's lifespan is limited, and it can degrade over time due to regular wear and tear. As a result, the sensing element may become less responsive, which can lead to diminished performance and heightened emissions.

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

Common Symptoms of a Faulty AFR Sensor

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

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

Increased exhaust fumes

When the AFR sensor delivers inaccurate signals to the ECU, it disrupts fuel management, potentially causing detrimental effects on engine performance and emissions. In cases where the engine operates normally or in a rich condition, but the AFR sensor incorrectly indicates a lean condition (by supplying a low voltage signal to the ECM), the ECM may compensate by injecting more fuel, leading to incomplete combustion and increased emissions. Conversely, if the engine is in a rich condition but the AFR sensor fails to signal the ECU, excess fuel may be injected, resulting in increasing emissions. Ultimately, these problems may cause the vehicle to fail an emission test and contribute to environmental pollution.

Decreased fuel efficiency

An AFR sensor that is slow to respond or has completely failed can send inaccurate data to the ECM, causing the engine to run with an excessively rich air-fuel mixture and resulting in increased fuel consumption. Consequently, the engine may compensate for this imbalance by consuming more fuel, further decreasing fuel efficiency.

Unstable Idling

At low engine speeds, the air-fuel mixture needs precise adjustment, making the signal from the air-fuel ratio sensor critical for the engine's idle process. A bad AFR sensor sends incorrect air-fuel mixture information to the ECM, causing irregular engine idling, rough running, or even stalling.

Poor performance while driving

If the AFR sensor malfunctions, the engine may hesitate or show poor acceleration, particularly when there are sudden throttle changes or during acceleration from a standstill. This can lead to sluggish performance and decreased responsiveness.

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Identify A Faulty AFR Sensor with the Innova Scan Tool

The Innova Scan Tool is a highly useful diagnostic device designed to quickly identify problems in a vehicle's systems. It is particularly effective in diagnosing and resolving issues related to the AFR sensor by retrieving and identifying Diagnostic Trouble Codes (DTCs) associated with it. In simpler terms, this tool efficiently aids in diagnosis of AFR sensor-related problems in vehicles, helping you easily pinpoint and address issues.