O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
The P0138 code indicates that the oxygen sensor (O2) located after the catalytic converter on Bank 1 (engine side with cylinder #1) is reporting a voltage signal that is higher than the expected range (typically above 1.0 volt). This sensor monitors the oxygen content in the exhaust to help the engine control module (ECM) adjust fuel mixture and verify catalytic converter efficiency. A high voltage reading suggests the sensor is stuck rich or there is a wiring issue, often leading to incorrect fuel trim adjustments and potential catalytic converter damage.
1. Most Common Causes
- Faulty oxygen sensor (Bank 1 Sensor 2)
- Short to voltage in sensor wiring or connector
- Exhaust leak upstream of the sensor
- Engine running rich (fuel system issue, faulty injector, etc.)
- Damaged catalytic converter causing contamination
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Poor fuel economy
- Failed emissions test
- Possible rough idle or hesitation
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data to confirm P0138 and check for related codes.
Step 2: Visually inspect the O2 sensor wiring and connector for damage, corrosion, or short to battery voltage.
Step 3: Using a multimeter, measure the sensor signal voltage at the ECM connector with engine running; compare to expected 0.1-0.9V cycling.
Step 4: If voltage is stuck high (above 1V), disconnect the sensor and check if voltage drops; if not, repair short to power; if it drops, replace the sensor.
Step 5: Check for exhaust leaks upstream and verify fuel trim data; if fuel trims are negative (rich), diagnose fuel system issues.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
You can drive short distances, but it may cause poor fuel economy and potential damage to the catalytic converter over time. It's best to diagnose and repair promptly.
How much does it cost to fix?▼
The cost typically ranges from $150 to $300, including parts (O2 sensor $50-$150) and labor (1-2 hours shop time).
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.