The downstream oxygen (O2) sensor (Sensor 2) is positioned in the exhaust system after the catalytic converter. It is the last sensor to read the exhaust gases before they exit the vehicle. This sensor’s primary output is a voltage signal that the engine control module (ECM) uses to monitor the overall health of the emissions system. Understanding these voltage readings is the most direct way to determine if the vehicle’s emissions equipment is functioning as designed.
Role in Catalytic Converter Monitoring
The downstream O2 sensor measures the effectiveness of the catalytic converter, unlike the upstream sensor, which adjusts the air-fuel mixture. The converter chemically reduces harmful pollutants. By measuring the oxygen content after this process, the sensor provides the ECM with data to assess efficiency.
The ECM constantly compares the downstream signal to the upstream sensor’s signal. If the catalytic converter is working correctly, it stabilizes the oxygen content in the exhaust stream. This stabilization confirms the converter is performing its chemical reactions effectively.
If the ECM detects the converter is not stabilizing the oxygen content, it registers a diagnostic trouble code. The most common code is P0420 (“Catalyst System Efficiency is Below Threshold”). This code results from the downstream sensor’s reading being too similar to the upstream sensor’s reading, signaling a lack of oxygen consumption.
Expected Normal Voltage Readings
A properly functioning downstream O2 sensor should display a voltage signal that is both high and relatively stable once the engine and catalytic converter are fully warmed up. The typical voltage range for a healthy sensor is consistently above 0.6 to 0.7 volts, often hovering near 0.8 or 0.9 volts. This high voltage reading occurs because the catalytic converter successfully consumes the remaining oxygen in the exhaust.
The sensor operates on the principle that low oxygen concentration generates a high voltage signal, and high oxygen concentration generates a low voltage signal. Since the converter uses oxygen to complete the combustion of pollutants, the exhaust gas exiting the converter should have very little free oxygen remaining. This low oxygen environment causes the sensor to maintain its steady, high voltage output.
The stability of the signal is important. Unlike the upstream sensor, which rapidly cycles between 0.1 and 0.9 volts to help the ECM maintain the ideal air-fuel ratio, the downstream sensor should show minimal fluctuation. This steady signal confirms the converter is acting as a buffer, smoothing out the rich and lean swings.
Diagnosing Issues from Abnormal Signals
Interpreting abnormal downstream O2 sensor signals involves recognizing two primary failure scenarios: a fluctuating signal or a signal stuck at an extreme voltage. If the downstream sensor’s voltage begins to cycle rapidly, mirroring the high-to-low swings of the upstream sensor, it strongly indicates a failed catalytic converter. This rapid cycling means the converter is no longer storing or consuming oxygen, triggering the P0420 efficiency code.
The second scenario involves a signal that is stuck, either consistently low near 0.1 volts or consistently high near 1.0 volt, showing no change regardless of engine operation. A stuck signal often points to a failure of the sensor itself. In this case, the sensor is not accurately reporting the oxygen content, which usually leads to a sensor-specific diagnostic code rather than an efficiency code.
A reading stuck low (near 0.1 volts) indicates a constant, high-oxygen (lean) condition, which can also be caused by a severe exhaust leak near the sensor. Conversely, a reading stuck high (near 1.0 volt) indicates a constant, low-oxygen (rich) condition. This may be caused by a sensor failure or an extreme engine condition, such as a severe misfire. Diagnosing the issue requires observing the sensor’s behavior to distinguish between a sensor failure and a catalytic converter failure.