P0137 – O2 Sensor Circuit Low Voltage Bank 1 Sensor 2

P0137 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) means the downstream oxygen sensor on Bank 1 is reporting a voltage that is lower than expected. This sensor monitors catalytic converter efficiency and helps the ECU fine-tune fuel trims. When its signal drops too low, you may experience poor fuel economy, emissions issues, or a failed inspection. This guide explains what P0137 means, the symptoms, causes, step-by-step diagnosis, and the best repair options.

What Does P0137 Mean?

P0137 is a generic OBD-II powertrain code that indicates **low voltage output from the downstream O2 sensor** (Bank 1 Sensor 2). This sensor sits after the catalytic converter and typically outputs between 0.1–0.9 volts.

If the ECU sees the voltage staying unusually low, usually below 0.1V for too long, it interprets this as a sensor or circuit issue — often linked to exhaust leaks, wiring problems, or a failing sensor.

Quick Reference

• OBD-II Family: P-Code (Powertrain)
• Scope: Generic
• System: Fuel & Emissions / Oxygen Sensor
• Difficulty Level: Easy–Moderate
• Estimated Repair Cost: €50–€250
• Last Updated: 4 December 2025

Real-World Example / Field Notes

A compact sedan came in with a check engine light and weak fuel economy. The downstream sensor was locked at ~0.05V while the catalytic converter was operating normally. After inspecting the harness, we found the sensor’s ground wire had chafed against the heat shield. Repairing the wire restored normal voltage and resolved P0137.

Another case involved an SUV with a small exhaust leak just ahead of the catalytic converter. Excess oxygen entering the system caused the sensor to read low voltage. After patching the leak, the downstream voltage stabilized and the code cleared.

Symptoms of P0137 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

• Poor fuel economy: ECU may adjust trims incorrectly.
• Check engine light: P0137 stored, possibly with related codes.
• Failed emissions test: Catalytic converter efficiency readings may be incorrect.
• Exhaust smell or noise: Often due to leaks affecting sensor readings.
• Slight hesitation: Rare but possible during deceleration.

Common Causes of P0137

Most Common Causes

• Faulty downstream O2 sensor (Bank 1 Sensor 2)
• Exhaust leaks near or before the catalytic converter
• Damaged O2 sensor wiring or connector
• Poor sensor ground or open circuit
• Contaminated sensor (coolant, oil, or silicone)

Less Common Causes

• Faulty catalytic converter affecting sensor readings
• Weak battery or charging system causing voltage instability
• ECU internal fault (rare)
• Aftermarket exhaust misalignment affecting sensor position
• High resistance in sensor heater circuit

Diagnosis: Step-By-Step Guide

P0137 diagnosis focuses on verifying sensor voltage, checking for exhaust leaks, and testing wiring integrity.

Tools You’ll Need

OBD-II scan tool with live data, multimeter, smoke machine (optional), wiring diagrams, and basic hand tools.

1. Read live O2 sensor voltage: Downstream sensor should normally switch slowly between ~0.1–0.9V.
2. Check for a stuck low reading: If voltage stays under 0.1V, suspect wiring or sensor failure.
3. Inspect exhaust system: Look for leaks near the mid-pipe, cat flange, or sensor bung.
4. Check wiring harness: Inspect for heat damage, melted insulation, or exposed wires.
5. Test sensor ground and signal: Use a multimeter to check for continuity and proper voltage.
6. Check for contamination: White or chalky buildup can indicate silicone contamination.
7. Perform a wiggle test: Move the harness while watching live data for voltage fluctuation.
8. Check battery voltage: Low system voltage can skew sensor reference values.
9. Verify catalytic converter operation: Compare upstream/downstream signals for proper efficiency.
10. Replace the O2 sensor if all wiring is confirmed good.

Pro Tip: If the downstream sensor voltage mirrors the upstream sensor exactly, the catalytic converter may not be working efficiently — suggesting a possible catalyst issue instead of a sensor failure.

Possible Fixes & Repair Costs

• Repair exhaust leaks: €50–€150
• Repair or replace damaged wiring: €40–€120
• Replace O2 sensor (Bank 1 Sensor 2): €80–€180
• Clean contaminated sensor connectors: €0–€20
• Replace catalytic converter (if needed): €250–€900+

Can I Still Drive With P0137?

Yes, the vehicle will still run, but fuel economy and emissions may suffer. Long-term driving with incorrect O2 readings can lead to catalyst stress or failed inspections. It’s best to diagnose and repair the issue soon.

Related Sensor Circuit Codes

Compare nearby sensor circuit trouble codes with similar definitions, fault patterns, and diagnostic paths.

• P0131 – O2 Sensor Circuit Low Voltage Bank 1 Sensor 1
• P0698 – Sensor Reference Voltage “C” Circuit Low
• P0652 – Sensor Reference Voltage “B” Circuit Low
• P0642 – Sensor Reference Voltage “A” Circuit Low
• P0192 – Fuel Rail Pressure Sensor Circuit Low
• P0112 – Intake Air Temperature Sensor 1 Circuit Low

Key Takeaways

• P0137 indicates low voltage from the downstream O2 sensor on Bank 1.
• Most common causes: sensor failure, wiring damage, or exhaust leaks.
• The vehicle may run normally but with poorer fuel economy and higher emissions.
• Fixing leaks or replacing the sensor usually resolves the issue quickly.

FAQ

What voltage should a downstream O2 sensor read?

Typically 0.1–0.9V. Downstream sensors switch slowly compared to upstream sensors.

Can an exhaust leak cause P0137?

Yes. Extra oxygen entering the exhaust lowers sensor voltage readings.

Is Bank 1 Sensor 2 the front or rear sensor?

Sensor 2 is always the downstream (after the catalytic converter) sensor.

Can P0137 be caused by a bad catalytic converter?

Indirectly, yes. A failing converter can distort downstream readings, but wiring or sensor issues are more common.

Will replacing the sensor always fix the code?

No. Always check wiring and exhaust leaks before installing a new sensor.