P0138
Low SeverityO2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Emissions System
P0138 — Instant Answer
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Severity: LOW SEVERITY
Meaning: Downstream O2 sensor bank 1 is stuck at a high voltage. Usually a failed sensor.
Safe to drive? Usually yes with caution, but confirm root cause before long trips.
Estimated repair cost: $25 - $80
Most common causes:
- 1. Failed O2 sensor (35-45% of cases)
- 2. Rich exhaust from upstream issue (10-25% of cases)
- 3. Short in sensor wiring (10-25% of cases)
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 Procedures for P0138
O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) Found 58 OEM service manual procedures.
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
Showing 8 of 58 procedures
OEM Diagnostic Flowchart
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Parts & Tools for This Job
Common parts shoppers buy for P0138 O2 Sensor Circuit High Voltage (Bank 1, Sensor 2).
Oxygen Sensor
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Catalytic Converter
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EVAP Purge Valve
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Gas Cap
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OBD2 scanner
Read codes and freeze-frame data before you guess at the fault.
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Check SpecificationsBattery tester
Check cranking and charging health before replacing expensive parts.
A weak battery can mimic alternator or starter problems.
Check SpecificationsMultimeter kit
Confirm voltage, continuity, and sensor power with one simple kit.
A meter is the cheapest way to rule out a bad circuit fast.
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Related Resources
Repair guides, symptom pages, and wiring diagrams related to P0138.
OEM Manual Evidence
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Related Symptoms
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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.
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Author & Review
Published by AllOEMManuals, founded by Lyndon Bedford, an automotive technician with 20 years of experience. He owned and operated a mechanic shop in Hurst, Texas from 2014 to 2022 and continues hands-on repair work.
Content may combine manual references, general repair templates, and AI-assisted drafting. Confirm the source, vehicle configuration, and applicability before working. Report corrections through our contact page.
Last updated: 2026-10-29