Air/Fuel Ratio Sensor (Upstream) – Accurate Mixture Control ✅

This sensor is vital for Toyota engines that use closed-loop fuel control. It “reads” the oxygen level in the exhaust and tells the ECU if the engine is running rich or lean. With that data, the ECU fine-tunes injector pulse and ignition strategy to keep the mixture near ideal for power, economy, and low emissions.

How It Works

Unlike a basic O2 sensor, an air/fuel sensor reacts faster and measures mixture more precisely. A built-in heater quickly brings the element to operating temperature, so feedback starts soon after cold start. The ECU monitors the signal and constantly corrects fuel trims to stabilize idle and throttle response.

Why You Need a Healthy Sensor

When this sensor is accurate, you get smoother driving, better fuel mileage, cleaner exhaust, and safer catalytic converter temperatures.

If It’s Failing or Broken ⚠️

A weak or dead sensor can cause poor MPG, rough idle, hesitation, black smoke, fuel smell, hard starting, and a check engine light. Long-term rich running may overheat and damage the catalytic converter, while lean signals can lead to misfires and higher NOx emissions.

Compatible Models

• Crown
• Crown/Majesta
• Mark X
• Reiz