P0139: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)
Updated November 13, 2026
The downstream oxygen sensor on bank 1 (after the catalytic converter) is reacting too slowly. The computer often tests it when you lift off the gas and the fuel is cut, expecting the reading to drop quickly. If it lags, P0139 sets. This sensor mostly monitors the converter, so the car usually drives normally.
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🔧 Want to check this yourself? An OBD2 scanner reads the code; models with live data also show the sensor readings used in the checks below. See code readers and scanners from $24.99 ↓ · Which scanner do I need?
Common causes
- An aging or contaminated downstream oxygen sensor
- An exhaust leak near the sensor letting in outside air
- Damaged wiring or a corroded connector at the sensor
- Oil or coolant being burned, which can coat and slow the sensor
Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.
Symptoms
- Check engine light, usually with no other symptoms
- Failed emissions or inspection test
Is it safe to drive?
Usually safe to drive with a steady light, but have it fixed to pass inspection. If the light flashes or the engine runs badly, stop driving and get it checked.
How to check it with a scanner
- Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
- Scan for other codes; misfire or fuel trim problems should be fixed first.
- With the engine off and cool, check the downstream sensor wiring and connector, and look for soot marks or listen at start-up for exhaust leaks near the sensor, keeping hands clear of the belts, fan and hot exhaust.
- With a scanner, watch the downstream sensor during a deceleration (with a passenger watching the screen): a healthy narrowband downstream sensor drops to a low voltage quickly when you lift off the gas.
- Check for oil or coolant loss, which can contaminate the sensor.
- After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.
A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.
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Questions
What does P0139 mean?
P0139 is the generic OBD2 code for "O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)." The downstream oxygen sensor on bank 1 (after the catalytic converter) is reacting too slowly. The computer often tests it when you lift off the gas and the fuel is cut, expecting the reading to drop quickly. If it lags, P0139 sets. This sensor mostly monitors the converter, so the car usually drives normally.
Is it safe to drive with P0139?
Usually safe to drive with a steady light, but have it fixed to pass inspection. If the light flashes or the engine runs badly, stop driving and get it checked.
How do I check P0139 myself?
Read the code and freeze-frame data with an OBD2 scanner, then: Scan for other codes; misfire or fuel trim problems should be fixed first. With the engine off and cool, check the downstream sensor wiring and connector, and look for soot marks or listen at start-up for exhaust leaks near the sensor, keeping hands clear of the belts, fan and hot exhaust. With a scanner, watch the downstream sensor during a deceleration (with a passenger watching the screen): a healthy narrowband downstream sensor drops to a low voltage quickly when you lift off the gas. Check for oil or coolant loss, which can contaminate the sensor.
Related codes
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