P0139 Code: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
🟢 Low severity · drivable
Safe to drive — this downstream sensor only monitors the catalytic converter.
What P0139 means
The bank 1 downstream oxygen sensor is switching too slowly. Because it sits after the catalytic converter and only monitors converter efficiency, a sluggish downstream sensor is usually just aging or contaminated — it won't affect drivability, but it can mask a converter problem and fail an emissions test.
Symptoms
- Check engine light on
- Usually no driving symptoms
Common causes
- most common: Aging or contaminated downstream oxygen sensor
- common: Exhaust leak near the sensor
- less common: Oil or coolant contamination of the sensor
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Replace the bank 1 downstream oxygen sensor | Yes | $60–200 |
| Repair an exhaust leak near the sensor | Mechanic | $50–200 |
Frequently asked questions
Is P0139 serious?
Not for drivability — the downstream sensor only monitors the converter. It mainly matters for emissions testing.
What causes P0139?
An aging downstream oxygen sensor is the usual cause.
How much does it cost to fix P0139?
Typically $60–$200 for a downstream oxygen sensor.
Related codes
P0136
P0136 Code: O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
P0133P0133 Code: O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
P0420P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
Related guide: MAF Sensor Replacement Cost →
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.