P0139
Oxygen Sensor Slow Response (Bank 1 Sensor 2)
The engine computer is not seeing the rear oxygen sensor change fast enough. This usually means the sensor is slow, the exhaust has a leak, or there is a wiring problem. It can also happen if the catalytic converter or fuel system is affecting exhaust readings.
- SEV
- 3/5
- DRIVE
- CAUTION
- DIY
- $20–$120
- SHOP
- $120–$450
Quick answer
AI-CITATION READYWhat it means
Can you drive with it?
Most common causes
- Aging or contaminated Bank 1 Sensor 2 oxygen sensor
- Exhaust leak near the rear oxygen sensor or before the sensor
- Damaged, corroded, or loose sensor wiring or connector
Typical repair cost
DIY usually runs $20–$120. Typical shop repair lands around $120–$450, depending on the root cause.
01 / Definition
P0139 means the powertrain control module has detected that Bank 1 Sensor 2, the downstream oxygen sensor after the catalytic converter, is switching voltage too slowly. The sensor signal is compared to expected changes during closed-loop operation, and the PCM expects a faster response than it is seeing. This can be caused by a worn oxygen sensor, exhaust leaks, damaged wiring, or less commonly a catalytic converter or engine running issue.
02 / Drive status
With caution. You can usually drive for a short time, but the problem should be diagnosed soon. If the check engine light is flashing, the engine runs badly, or fuel mileage drops sharply, stop driving and get it checked right away.
03 / Symptoms
- Check engine light on
- Reduced fuel economy
- Failed emissions test
- Possible rough idle if another engine problem is present
- No obvious drivability symptoms in some cases
04 / Causes
| 1 | Aging or contaminated Bank 1 Sensor 2 oxygen sensor | high |
|---|---|---|
| 2 | Exhaust leak near the rear oxygen sensor or before the sensor | high |
| 3 | Damaged, corroded, or loose sensor wiring or connector | high |
| 4 | Poor fuel control, misfires, or engine running issues affecting exhaust readings | medium |
| 5 | Catalytic converter efficiency or flow problems | medium |
| 6 | PCM issue or incorrect software calibration | low |
05 / Diagnostic sequence
- 01Check for additional codes, especially misfire, fuel trim, heater circuit, or catalyst codes.
- 02Inspect the Bank 1 Sensor 2 wiring, connector, and harness for heat damage, corrosion, looseness, or broken wires.
- 03Look for exhaust leaks at the manifold, pipes, and joints near Bank 1 and the rear oxygen sensor.
- 04Review live data for Bank 1 Sensor 2 response and compare it with Bank 1 Sensor 1 and fuel trim values.
- 05Verify that the engine is running correctly, with no misfires, vacuum leaks, or fuel delivery problems.
- 06Test the oxygen sensor signal and heater circuit according to the service manual.
- 07If needed, replace the sensor and clear codes, then confirm the repair with a road test and scan data.
06 / Repairs
| 1 | Repair exhaust leaks and damaged wiring or connectors | $20–$200 |
|---|---|---|
| 2 | Replace the Bank 1 Sensor 2 oxygen sensor | $50–$180 |
| 3 | Repair underlying engine issues such as misfires, vacuum leaks, or fuel system problems | $50–$400 |
| 4 | Replace the catalytic converter if testing confirms it is affecting sensor response | $400–$1,500+ |
07 / Related codes
08 / FAQ
Is P0139 always a bad oxygen sensor?
No. The sensor is common, but exhaust leaks, wiring faults, and engine problems can also cause this code.
Which sensor is Bank 1 Sensor 2?
It is the downstream oxygen sensor on the side of the engine with cylinder 1, located after the catalytic converter.
Will replacing the oxygen sensor fix P0139?
Sometimes, but not always. The sensor should be tested and the exhaust system checked first so you do not replace parts unnecessarily.
Can a catalytic converter cause P0139?
Yes, but it is less common than a sensor, wiring, or exhaust leak problem. Converter issues should be confirmed with testing.
09 / Source and method
- DATA BASIS
- OBD-II REFERENCE + OBD2.HELP
- METHOD
- STATIC VALIDATION
- SAFETY
- INFORMATIONAL
This page combines OBD-II diagnostic reference data with OBD2.help generated diagnostic guidance for code meaning, likely causes, and repair direction.
Publishing uses deterministic schema and build validation, plus manual spot checks on representative pages before release.
Safety-critical diagnosis and repairs should be confirmed with a qualified mechanic, especially when the vehicle is misfiring, overheating, or losing power.