What Does P0139 Mean?
The ECM has detected that the downstream oxygen sensor (Bank 1 Sensor 2), located after the catalytic converter, is responding too slowly when switching between rich and lean states. This sensor monitors catalytic converter efficiency, and a slow response time indicates the sensor is aging or contaminated, which can impair the ECM's ability to accurately evaluate catalyst performance. The driver may notice slightly increased emissions and potentially a marginal decrease in fuel economy.
Common Causes
40%
Aging or degraded downstream O2 sensor with slow response characteristics
25%
O2 sensor contaminated by oil consumption, coolant intrusion, or use of RTV silicone
15%
Exhaust leak upstream of the downstream sensor affecting sensor readings
15%
Catalytic converter beginning to lose efficiency, altering gas composition reaching the sensor
5%
Wiring or connector issue causing signal degradation to the O2 sensor
Diagnostic Steps
1
Monitor live data for the Bank 1 Sensor 2 O2 signal. At steady cruise, the downstream sensor should show a relatively stable voltage (0.5-0.7V) if the catalyst is working properly. Look for the sensor's transition time between rich (>0.45V) and lean (<0.45V) — it should respond within 100ms.
2
Compare freeze frame data to determine if the slow response occurs during specific conditions (acceleration, deceleration, or steady-state).
3
Inspect the downstream O2 sensor for contamination — white deposits indicate coolant contamination, dark sooty deposits indicate rich running or oil burning.
4
Check for exhaust leaks between the catalytic converter and the downstream sensor location. Even small leaks can introduce fresh air that dilutes exhaust gas readings.
5
If the sensor appears visually clean and there are no exhaust leaks, perform an intrusive test by inducing a rich condition (propane) and observing how quickly the downstream sensor responds compared to the upstream sensor.
Estimated Repair Cost
$100 - $350
Parts + labor, varies by vehicle and location
The ECM has detected that the downstream oxygen sensor (Bank 1 Sensor 2), located after the catalytic converter, is responding too slowly when switching between rich and lean states. This sensor monitors catalytic converter efficiency, and a slow response time indicates the sensor is aging or contam...
The most common cause of P0139 (Oxygen Sensor Slow Response, Engine Back A, Downstream of Catalytic Converter) is: Aging or degraded downstream O2 sensor with slow response characteristics
Typical repair costs for P0139 range from $100 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to drive. This code primarily affects emissions monitoring accuracy. There is no power loss or driveability concern. However, the MIL will remain illuminated and the vehicle will fail emissions testing until repaired.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0139 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Exhaust
Difficulty
Type
Generic (SAE)
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