DTC P0138 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 2).

🔧 DTC P0138 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

Meaning:
The downstream O₂ sensor (Bank 1, Sensor 2) is sending a higher-than-normal voltage signal to the ECM (usually > 0.9V for too long).
This sensor is after the catalytic converter and monitors catalyst efficiency.

✅ Common Causes

1. Faulty O₂ sensor (B1S2)

2. Short to voltage in the signal wire

3. Exhaust leak near catalytic converter

4. Rich running condition (too much fuel)

5. Damaged wiring or connector – rubbed, melted, corroded

6. Faulty ECU (rare)

🔎 Step-by-Step Diagnosis

1. Scan live data

Check Bank 1 Sensor 2 voltage:

Normal: 0.1–0.9V (slow switching)

In P0138 case: Stuck high ~0.9–1.0V

If stuck high → wiring or sensor issue.

2. Check wiring & connector

Look for:

Melted wires (common near exhaust)

Short to battery voltage

Water/oil in connector

Damaged ground wire

👉 Repair wiring first if damaged.

3. Check exhaust for leaks

Leak before Bank 1 Sensor 2 can cause high readings.

4. Test the O₂ sensor

Unplug sensor → monitor voltage

If voltage still high → wiring short

If voltage drops to 0V → sensor faulty

5. Check fuel system

A rich mixture can cause high downstream voltage:

Leaking injector

High fuel pressure

Faulty MAF

Stuck purge valve

If vehicle has misfire or black smoke → check fuel trim data.

🛠 Common Fixes

💥 Replace O₂ sensor Bank 1 Sensor 2
💥Repair shorted wiring harness
💥 Fix exhaust leak
💥 Correct rich running conditions
💥 Clean/replace MAF if needed
💥 Repair connector corrosion

🧪 After Repair

Clear the code

Road test

Check live data to ensure B1S2 voltage is normal

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