P0138
Low SeverityO2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Emissions System
What Does P0138 Mean?
Downstream O2 sensor bank 1 is stuck at a high voltage. Usually a failed sensor.
Common Fix
Replace downstream O2 sensor
Estimated Cost (DIY)
$25 - $80
Symptoms
- ⚠️Check engine light
- ⚠️Failed emissions
- ⚠️No performance symptoms
Common Causes
Failed O2 sensorlikely
Rich exhaust from upstream issuepossible
Short in sensor wiringpossible
How to Diagnose P0138
- 1Check upstream fuel trim — if normal, sensor is likely bad
- 2Monitor sensor voltage
- 3Inspect wiring for shorts
- 4Replace sensor
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Frequently Asked Questions
- Is P0138 serious?
- Not in terms of driveability but it will fail emissions. A mechanic might misdiagnose this as a catalytic converter issue.
- Can P0138 damage the catalytic converter?
- No — the downstream sensor only monitors the cat, it does not control fuel mixture.