O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

What is P0137?

P0137 indicates the downstream oxygen sensor (Bank 1 Sensor 2) is reporting a voltage lower than expected, which means the powertrain control module cannot properly monitor catalytic converter efficiency. This is a medium-severity issue that can cause failed emissions tests and reduced fuel economy. Based on owner reports, this code frequently appears on modified or performance-oriented vehicles like the Subaru WRX and Ford Mustang.

Common Causes

• ●Faulty or failing downstream oxygen sensor (Bank 1 Sensor 2) is the most likely cause, as this code directly indicates a sensor circuit problem.
• ●Exhaust system modifications, such as aftermarket headers or performance mufflers, which were mentioned in several owner reports, can interfere with sensor readings.
• ●Wiring issues or connector problems in the oxygen sensor circuit, including corrosion or damage from road debris or accidents.

Symptoms You May Notice

• ●Illuminated check engine light is the primary symptom owners notice, often without immediate drivability issues.
• ●Potential decrease in fuel economy, as the engine may not be running at optimal efficiency without proper downstream oxygen feedback.
• ●Failed emissions test during inspection, since the downstream sensor is critical for monitoring catalytic converter performance.

How to Fix It

• ●Replace the downstream oxygen sensor (Bank 1 Sensor 2), which is the most direct fix for this circuit code (cost $100-$300, moderate difficulty).
• ●Inspect and repair wiring or connectors to the oxygen sensor, especially if there is visible damage or corrosion (cost $50-$200, moderate difficulty).
• ●Address any exhaust system modifications that may be causing improper sensor readings, such as ensuring proper installation of aftermarket headers or mufflers (cost varies widely, high difficulty).