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
Related Symptoms
Common Causes
How to Diagnose P0138
- 1Check upstream fuel trim — if normal, sensor is likely bad
- 2Monitor sensor voltage
- 3Inspect wiring for shorts
- 4Replace sensor
OEM Diagnostic Flowchart
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Read live data and freeze frame for P0138 to pinpoint the cause before buying parts.
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Related Resources
Repair guides, symptom pages, and wiring diagrams related to P0138.
OEM Manual Evidence
Browse →A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL Denali AWD V8-6.2L Service Manual | Operation CHARM Operation CHARM : Car...
A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL Denali 2WD V8-6.2L Service Manual | Operation CHARM Operation CHARM : Car...
A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL 4WD V8-6.0L Service Manual | Operation CHARM Operation CHARM : Car repair...
A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL 4WD V8-5.3L Service Manual | Operation CHARM Operation CHARM : Car repair...
Related Symptoms
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Exact Repair Workflows
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Related Codes
General fault with the downstream (post-cat) O2 sensor on bank 1. This sensor monitors catalytic converter efficiency.
Downstream O2 sensor on bank 1 is reading too low. Could indicate a failing sensor or a lean exhaust condition.
The catalytic converter on bank 1 is not converting pollutants efficiently. This is one of the most common DTCs.
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.
Related Repair Guides
If your vehicle has triggered P0138, these DIY repair guides may help: