P0138 Code: Meaning, Symptoms, Causes & Fix

Overview

Code P0138 indicates a fault in O2 Sensor Circuit High Voltage (Bank 1 Sensor 2), typically within powertrain controls.

Symptoms

reduced performance
increased fuel use
check engine light

Common causes

Disconnected or loose connector affecting o2 sensor circuit high voltage (bank 1 sensor 2)
Open circuit in wiring
Connector pin damage/corrosion or harness stress near related components

How to diagnose P0138

Confirm P0138 and capture freeze-frame data with a professional scan tool.
Inspect wiring, connectors, and grounds related to o2 sensor circuit high voltage (bank 1 sensor 2) for obvious defects.
Use a continuity test from module pin to component to identify opens/high resistance.
Check for related DTCs to identify the root-cause chain before replacing parts.
After repair, clear codes and confirm readiness monitors complete without recurrence.

Severity of code P0138

Moderate: This code is moderate severity: the vehicle may still operate, but fault progression can increase repair cost over time.

Can you drive with P0138?

Most vehicles can be driven short-term with P0138, but postpone heavy load/highway driving until repaired.

If the warning lamp is flashing or drivability/safety is affected, avoid driving and diagnose immediately.

How to fix it (step-by-step)

Repair obvious wiring/connector faults first to prevent repeat parts replacement.
Address the root fault mode (open) in the o2 sensor circuit high voltage (bank 1 sensor 2) circuit/system.
Apply the required relearn/adaptation procedure if specified by service information.
Road-test and verify P0138 remains cleared under the conditions that originally set the code.

Fix options

Clear corrosion and protect terminals.
Restore continuity and verify pin tension.
Clear DTCs and verify with live data and completed drive cycle.

Typical repair cost

$200-$900

Common mistakes when diagnosing P0138

Replacing parts without verifying voltage, ground, and signal integrity first.
Ignoring related stored/pending codes that indicate upstream faults.
Skipping post-repair verification drive cycle and readiness checks.

P0138 in common vehicles

P0138 appears across makes such as Mazda, Subaru, Lexus, Jeep. Failure patterns differ by platform: wiring routes, software calibration, and component supplier revisions can change root cause.

Frequently asked questions

What causes P0138?

P0138 is commonly set by open faults affecting o2 sensor circuit high voltage (bank 1 sensor 2).

How much does it cost to fix P0138?

For P0138, repair costs are typically $200-$900 depending on exact root cause, labor rates, and part quality.

Can I clear P0138 without repair?

You can clear P0138 temporarily, but the code usually returns until the underlying fault is corrected.