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

What error code P0139 is trying to tell you

The diagnostic trouble code P0139 stands for “O2 Sensor Circuit Slow Response (Bank 1, Sensor 2).” That means the downstream oxygen sensor, near the back of the car on the exhaust, is taking too long to respond to expected changes in the air-fuel mixture. The sensor measures the catalyst's efficiency by taking oxygen in the exhaust gas after it has passed through the catalytic converter. 

Why this issue matters

If this oxygen sensor is slow, that means it might not be measuring levels correctly. As a result, the engine control module won't be able to properly assess if the catalytic converter is working correctly. This issue may result in increased emissions and poor fuel economy, and it could ultimately cause your car to fail an emissions test.

Can I safely drive with this error code?

In most cases, it is safe to drive short distances with code P0139. Drivability may not be immediately affected, but the problem should be diagnosed and corrected to prevent long-term damage to the emissions system. The issue also could result in a failed emissions test, which may prevent you from registering your car in certain states. 

Common symptoms

• Illuminated check engine light
• Reduced fuel efficiency
• Failed emissions test
• Occasional engine hesitation (in rare cases)
  

How is the error code P0139 diagnosed?

A technician will begin by scanning the car's onboard computer for trouble codes to confirm the issue. Next, the technician will inspect where the oxygen sensor wiring and connectors where they mount up to the exhaust system for damage or corrosion. Live data from the oxygen sensor will be checked to assess response time and confirm that it is, in fact, slow. In some cases, the technician may use a propane enrichment test or introduce a vacuum leak to verify sensor reaction.

How do I fix it?

If wiring issues or connector damage are found, these components can be repaired, but a replacement is the most common fix. The catalytic converter might also be faulty and need replacement, but this is a rare case. Once the issue is resolved, the code is cleared and the system is retested to make sure it's working properly. 

Related reading

Follow the link below to reach our diagnostic check engine trouble codes catalog:
On-board diagnostics (OBD-II) trouble code directory