P0137 Code: O2 Sensor Low Voltage – Causes & Fixes

What Does P0137 Mean?

The P0137 diagnostic trouble code stands for “O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2).” This code is triggered when your vehicle’s engine control module (ECM) detects that the oxygen sensor on Bank 1, Sensor 2 (the downstream sensor after the catalytic converter) is producing a voltage signal that is lower than the expected range.

Oxygen sensors measure the amount of unburned oxygen in the exhaust stream and send this information to the ECM, which uses it to adjust the air-fuel mixture for optimal engine performance and emissions control. When the sensor voltage drops below approximately 0.1 volts for an extended period, the ECM logs code P0137.

Bank 1 refers to the side of the engine that contains cylinder #1, while Sensor 2

Common Symptoms

• Check Engine Light: The most obvious sign—your dashboard warning light illuminates
• Reduced Fuel Economy: The engine may run rich (too much fuel), consuming more gas than normal
• Rough Idle: Engine may stumble or feel uneven at stops
• Hesitation During Acceleration: Sluggish throttle response or momentary power loss
• Rotten Egg Smell: Unburned fuel in the exhaust creates a sulfur-like odor
• Black Smoke from Exhaust: Sign of running too rich
• Catalytic Converter Overheating: In severe cases, you may notice heat radiating from under the vehicle

Possible Causes (Ranked by Frequency)

1. Faulty O2 Sensor (Most Common): The sensor itself has failed and no longer produces proper voltage signals. This accounts for roughly 70% of P0137 cases.
2. Wiring Issues: Corroded, damaged, or loose wiring between the sensor and ECM prevents proper signal transmission. Check connectors for corrosion or water damage.
3. Exhaust Leaks: Cracks or holes in the exhaust system before Sensor 2 allow outside air to enter, skewing oxygen readings and causing low voltage signals.
4. ECM Connector Problems: Poor connections at the engine control module can interrupt the sensor signal.
5. Fuel System Issues: A stuck fuel injector or failing fuel pressure regulator causing a rich condition can indirectly trigger this code.
6. Engine Control Module Failure: Rarely, the ECM itself is defective and misinterprets a normal sensor signal as low voltage.

Diagnostic Steps

Step 1: Retrieve and Document the Code

Use an OBD-II scanner to confirm code P0137 and check for any additional codes that might provide clues (such as P0171 for fuel trim issues or exhaust leak codes).

Step 2: Visual Inspection

• Locate the downstream O2 sensor on Bank 1 (typically under the vehicle, mounted in or near the exhaust manifold)
• Inspect the sensor connector for corrosion, moisture, or loose pins
• Check the wiring harness for cuts, abrasions, or damage
• Look for exhaust leaks between the engine and the sensor location

Step 3: Check Sensor Voltage

With the engine running, use a digital multimeter to measure the O2 sensor voltage:

• Healthy sensors typically fluctuate between 0.1 and 0.9 volts
• A sensor stuck below 0.1 volts confirms a faulty sensor
• If voltage is normal, the problem may be in the wiring or ECM

Step 4: Inspect Exhaust System

Check for exhaust leaks by:

• Visually inspecting the exhaust pipe and manifold for cracks
• Listening for hissing sounds while the engine idles
• Using soapy water around suspected leak areas (bubbles indicate leaks)

Step 5: Test Wiring and Connectors

Using a multimeter in continuity mode, test the wiring between the sensor and ECM. Resistance should be minimal (less than 1 ohm). High resistance or no continuity indicates a wiring problem.

Step 6: Replace or Repair

Based on your findings:

• If sensor voltage is low: Replace the O2 sensor
• If wiring is damaged: Repair or replace the harness
• If exhaust leaks are found: Repair or replace the affected component
• If connectors are corroded: Clean or replace them

Repair Cost Estimates

Note: Labor costs vary by location and shop. Dealerships typically charge more than independent mechanics. Many shops charge $80–$150 per hour for diagnostic work.

Can I Still Drive?

Severity: Low

Code P0137 is generally considered a low-severity code, and you can typically continue driving your vehicle safely in the short term. However, you should address it soon to avoid additional problems:

• Safe to Drive: Yes, but with caution. The vehicle will remain drivable, though fuel economy will suffer.
• Emissions: Your vehicle may fail an emissions test with this code active.
• Engine Damage Risk: Low risk of immediate engine damage, but prolonged rich running conditions can damage the catalytic converter over time, leading to much more expensive repairs ($800–$2,000+).
• Recommendation: Schedule a repair within the next few days to weeks. Don’t ignore it indefinitely.

Frequently Asked Questions

Can I clear P0137 without fixing it?

Yes, you can clear the code with a scanner, but it will return within a few driving cycles if the underlying problem isn’t fixed. Clearing the code without repair is only a temporary solution and doesn’t address the root cause.

What’s the difference between Sensor 1 and Sensor 2?

Sensor 1 (upstream) is located before the catalytic converter and is critical for fuel mixture control. Sensor 2 (downstream) monitors the catalytic converter’s efficiency. A faulty Sensor 2 has less immediate impact on drivability but still affects emissions and fuel economy.

Is P0137 the same as P0138 or P0139?

No. P0137 is low voltage, P0138 is high voltage, and P0139 is slow response time—all for the same sensor. Each code points to a different type of malfunction and may have different causes.

Can a bad catalytic converter cause P0137?

Indirectly, yes. A failing catalytic converter can cause exhaust backpressure issues that affect oxygen sensor readings. However, a bad catalytic converter typically triggers its own codes (P0420 or P0430) first. Always check for exhaust leaks and sensor wiring before assuming the converter is the problem.