P0139 - O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)

Bank 1 downstream oxygen sensor is switching too slowly.

Code Overview

Bank 1 downstream oxygen sensor is switching too slowly.

Common Causes

• Aging downstream oxygen sensor on bank 1 sensor 2
• Exhaust leak affecting downstream sensor readings
• Catalyst efficiency problem slowing sensor response
• Rich or lean running condition affecting the monitor

Symptoms

• Check-engine light with little or no drivability change
• Failed emissions readiness or inspection
• Catalyst monitor may not complete normally

Diagnostic Steps

1. Watch bank 1 sensor 2 response on a scan tool after the engine is fully warm
2. Check for exhaust leaks ahead of the downstream sensor
3. Look for companion fuel-trim, misfire, or catalyst codes first
4. Compare downstream sensor behavior to catalyst and fueling conditions before replacing parts

Repair Difficulty

Moderate

General difficulty estimate for the most common repair path.

Likely Repairs & Cost Guides

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Related OBD Codes

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Next Steps

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