Emission controls · Powertrain · generic (SAE-defined)
P0139: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
P0139 means the downstream oxygen sensor after the bank 1 catalytic converter is reacting too slowly to changes in the exhaust, usually a sign of an aging sensor.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0139 sets
To test the bank 1 sensor 2 oxygen (O2) sensor, the powertrain control module (PCM) often watches its signal during deceleration fuel cutoff, when the exhaust suddenly turns lean. If the sensor takes too long to drop from a rich to a lean reading, the PCM concludes it is slow and sets P0139. Test conditions and timing are manufacturer-specific.
Common causes of P0139
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Aged or contaminated bank 1 sensor 2 oxygen sensor
- Exhaust leak near the sensor
- Weak sensor heater
- Oil or coolant contamination from engine wear
- Wiring resistance or poor connector contact
P0139 symptoms
- Check engine light on
- Catalyst readiness monitor may not complete
- Usually no noticeable drivability symptoms
How P0139 is diagnosed
- Scan for codes; address heater codes (P0141) first since a cold sensor responds slowly.
- Inspect the exhaust for leaks near the downstream sensor.
- With the engine warm, record the sensor during a coast-down from highway speed and note how fast the reading falls.
- Compare its response to the bank 2 downstream sensor if equipped.
- Check the connector and wiring for corrosion or high resistance.
Typical fixes for P0139
- Replace the bank 1 sensor 2 oxygen sensor
- Repair exhaust leaks
- Repair heater circuit faults
- Correct oil burning or coolant leaks contaminating the sensor
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
P0139 FAQ
What does code P0139 mean?
P0139 means the downstream oxygen sensor after the bank 1 catalytic converter is reacting too slowly to changes in the exhaust, usually a sign of an aging sensor. The standard definition is "O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)."
Can I drive with a P0139 code?
Yes, the car normally runs as usual. Because the catalyst monitor relies on this sensor, repair it before an emissions inspection.
How is P0139 different from P0133?
P0133 refers to a slow upstream sensor ahead of the catalytic converter on bank 1, while P0139 refers to the downstream sensor after the converter on the same bank.
Will P0139 lower my gas mileage?
Usually not by much, because the downstream sensor has limited influence on fueling. Its main role is monitoring the converter.
Keep researching
More emission controls codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.