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

P0139 code definition

P0139 code means O2 Sensor Circuit Slow Response (Bank 1, Sensor 2).

The O2 sensors are used to measure the amount of oxygen in your exhaust system, which can then be used to determine how much fuel should be added to keep your engine running at its most efficient level.

If you get this code, either one or both of your O2 sensors have likely failed or become disconnected. In some cases, however, it could also mean something is wrong with your vehicle’s computer system (the ECU).

Possible causes of the P0139 code

Common causes of the P0139 code, indicating a slow response from the downstream oxygen sensor, include:

Faulty oxygen sensor.
Wiring issues or connector damage.
Vacuum leaks in the exhaust system.
Contamination on the sensor (oil or coolant).
Exhaust system leaks.
Catalytic converter problems.
Engine mechanical issues (misfires, timing problems).
Faulty Engine Control Module (ECM).

Symptoms of a P0139 code

The check engine light will be illuminated.
The engine may run rough or not start at all.
It may stall while driving or just after starting the vehicle due to poor fuel economy caused by increased fuel consumption when an oxygen sensor fails to operate correctly

Diagnostic procedure for your vehicle

Check for codes
Check for vacuum leaks
Check for exhaust leak
Check for bad O2 sensor
Check for bad ground
Check fuel pressure and injector operation (manual)

What causes a slow response in the O2 sensor?

Possible causes include a faulty oxygen sensor, wiring issues, vacuum leaks, or a deteriorating catalytic converter affecting sensor performance.

How does the downstream oxygen sensor function?

The downstream sensor measures the efficiency of the catalytic converter by analyzing oxygen levels in the exhaust after it has passed through the converter.

Why is a slow response concerning?

A slow response may indicate a decline in the catalytic converter’s efficiency, potentially leading to increased emissions and reduced fuel efficiency.

Can a dirty or contaminated sensor cause P0139?

Yes, contaminants like oil or coolant on the sensor can hinder its response. Regular maintenance can help prevent such issues.

How is P0139 typically repaired?

Solutions may involve replacing the oxygen sensor, fixing wiring issues, addressing vacuum leaks, or replacing a failing catalytic converter.

How much does it cost to fix a P0139 code?

Costs vary based on the cause. Replacing an oxygen sensor may cost $100 to $300, while catalytic converter replacement can be more expensive.

Here’s a video that shows you how to fix P0139 engine code.