P0138 Code: O2 Sensor High Voltage (B1S2) — Causes & Fix
P0138 means the downstream oxygen sensor (bank 1, sensor 2) is reporting a persistently high voltage — an unusually rich reading after the converter.
- 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 a shorted signal wire, a contaminated sensor, or a genuinely rich condition/fuel in the exhaust. P0138 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
- Possible rich running or fuel smell
- May fail emissions
How to fix it
- 1
Inspect wiring and for rich causes
1. Check the sensor signal wire for a short to voltage. 2. Rule out a leaking injector or high fuel pressure.
- 2
Replace the sensor
1. Replace a contaminated downstream O2 sensor. 2. Clear the code.
Frequently asked questions
Can a rich engine cause P0138?
Yes — raw fuel reaching the rear sensor pins its voltage high. Check for rich causes, not just the sensor.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.