P0139 Code: O2 Sensor Slow Response (B1S2) — Causes & Fix
P0139 means the downstream oxygen sensor (bank 1, sensor 2) is responding too slowly — the post-catalyst monitor is sluggish.
- Applies to
- All OBD-II vehicles (any make or model)
- Est. repair cost
- $40 – $300
- Time to fix
- 30 minutes – 2 hours
What it means
Causes include an aged downstream O2 sensor, contamination, or an exhaust leak. P0139 is a generic OBD-II powertrain code defined in the SAE J2012 standard, so it applies to virtually any 1996-or-newer petrol car or light truck regardless of make. Fix the underlying fault, then clear the code with an OBD-II scan tool — it will return if the problem isn't resolved.
Symptoms
- Check engine light on
- Little drivability effect
- May affect emissions monitoring
How to fix it
- 1
Check for exhaust leaks
1. Inspect the exhaust around the downstream sensor.
- 2
Replace the sensor
1. Replace a lazy downstream O2 sensor. 2. Clear the code and complete a drive cycle.
Frequently asked questions
Is P0139 a converter problem?
Not directly — it is a downstream sensor response fault. A worn converter usually shows as P0420.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.