OBD2 Code P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
The P0138 code means O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). Estimated repair cost: $100 – $350.
✅ OK TO DRIVE (short term). Have it diagnosed at your next convenience. Monitor for new symptoms.
📋 Diagnostic Reference
| OBD2 Code | P0138 |
| Category | Powertrain |
| Severity | Check Soon |
| Repair Cost | $100 – $350 |
| Can You Drive? | ✅ Yes — diagnose at your convenience |
🔍 Symptoms
- Check Engine Light on
- no noticeable driving symptoms
- may fail emissions
⚠️ Most Likely Causes (in order of probability)
- Failed rear O2 sensor
- shorted O2 sensor wiring
- rich fuel mixture
- exhaust leak near sensor
🛠️ Diagnostic Procedure
Step 1: Check for exhaust leak near sensor — fresh air dilutes reading
Check for exhaust leak near sensor — fresh air dilutes reading
Step 2: Test O2 sensor voltage with scan tool
Test O2 sensor voltage with scan tool
Step 3: Inspect wiring for shorts to 12V
Inspect wiring for shorts to 12V
Step 4: Replace rear O2 sensor.
Replace rear O2 sensor.
ASE Certified Review: This diagnostic guide has been reviewed by an ASE-certified master technician. Diagnostic steps follow industry best practices per SAE J1979 and OEM service procedures. Always verify with a qualified mechanic before performing repairs.
Technician's Notes: Bank 1 Sensor 2 is the downstream sensor after the cat. It only monitors cat efficiency.
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Data Sources: Diagnostic information compiled from SAE J1979 OBD2 standards, OEM service manuals, and ASE master technician field experience. Reviewed for technical accuracy. Repair cost ranges are national estimates and vary by location. Always verify with a qualified mechanic.