O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
What does P0139 mean?
The P0139 code means the vehicle's onboard diagnostic system has detected: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2). This is a low severity code.
Common Symptoms
- Check Engine Light on
- Slightly reduced fuel economy
- Possible slight increase in emissions
- No noticeable drivability issues
- Catalyst monitor may not complete
Probable Causes (Ranked by Likelihood)
- Aging downstream O2 sensor with slow response time Very High
- Exhaust leak before the downstream sensor High
- Contaminated O2 sensor from oil or coolant in exhaust Moderate
- Wiring issue causing degraded signal Moderate
- Failing catalytic converter affecting sensor readings Low
Step-by-Step Diagnostic Procedure
-
Use a scan tool to monitor bank 1 sensor 2 O2 voltage during a steady cruise at 2,000 RPM. The downstream sensor should show a relatively steady voltage (0.5-0.7V) if the catalyst is working. Slow transitions indicate a lazy sensor.
-
Compare the downstream sensor switching rate to the upstream sensor. The downstream should switch much less frequently than the upstream if the catalyst is functioning.
-
Check for exhaust leaks between the catalytic converter and the downstream sensor location. Use a smoke machine or visual inspection.
-
Inspect the O2 sensor for contamination (white residue = coolant, oily residue = burning oil).
-
Check wiring and connector for corrosion, damage, or high resistance that could slow the signal.
Frequently Asked Questions
Can I drive with code P0139?
Yes, you can drive normally. The downstream O2 sensor monitors catalytic converter efficiency. A slow response will not affect engine performance but may indicate the sensor or catalyst is aging.
Could P0139 actually be a catalytic converter problem?
Possibly. A deteriorating catalytic converter can cause the downstream O2 sensor to show unusual patterns that mimic a slow sensor. If replacing the sensor does not fix the code, have the catalytic converter tested.
How long do O2 sensors last?
O2 sensors typically last 60,000 to 100,000 miles. The downstream sensor often lasts longer because it is exposed to cleaner exhaust after the catalytic converter. If your vehicle has high mileage, replacing the sensor is a reasonable first step.