P0137: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) Explained

Quick answer

• What this code means: The post-catalytic converter oxygen sensor (Bank 1 Sensor 2) is reporting a consistently low voltage, indicating a possible rich condition or a sensor/circuit fault.
• Severity: Moderately severe. While your car might run, emissions will likely be poor, and fuel economy could suffer. Extended driving could damage the catalytic converter.
• Most common cause: A faulty downstream oxygen sensor (Bank 1 Sensor 2).

What the P0137 code means

The P0137 code stands for "O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)". In simpler terms, your car's Powertrain Control Module (PCM) or Engine Control Unit (ECU) has detected that the oxygen sensor located after the catalytic converter on Bank 1 (the side of the engine with cylinder #1) is sending a voltage signal that is consistently too low. This low voltage implies that the sensor is detecting an abundance of oxygen in the exhaust, which usually suggests a very lean exhaust condition or, more commonly, an issue with the sensor itself or its circuit.

Oxygen sensors measure the oxygen content in the exhaust gases to help the engine's computer adjust the air-fuel mixture for optimal combustion and emissions. Sensor 1 (upstream) primarily regulates air-fuel, while Sensor 2 (downstream) monitors the catalytic converter's efficiency. A consistently low voltage from Sensor 2 indicates that it's not oscillating as it should, or it