AutoFixDB - Free OBD-II diagnostic trouble code lookup. Find causes, symptoms, diagnostic steps and repair solutions.

Overview

P0138 indicates high voltage in the oxygen sensor circuit for Bank 1, Sensor 2 (downstream O2 sensor), often due to sensor or wiring issues and can affect emissions performance

Common Symptoms

Check Engine Light illuminated
Reduced fuel economy
Rough idling
Poor acceleration or performance
Failed emissions test

Most Common Causes

Faulty Bank 1 Sensor 2 oxygen sensor high

Sensor failure is frequently the root cause of sustained high voltage readings

Wiring or connector issues (short to voltage, corrosion, damage) high

Pinched or damaged harness can pull signal high

Exhaust leaks before the sensor medium

Leaks can alter oxygen readings and mimic high voltage conditions

ECM malfunction low

Rare, but internal control module faults can misinterpret sensor signals

Diagnostic Steps

Retrieve and confirm P0138 with an OBD-II scanner

Expected: Confirmed P0138 and any related codes

Interpretation: Verifies code presence and contextual data for further diagnosis
Visual inspection of Bank 1 Sensor 2, wiring harness, and connectors

Expected: No obvious damage, corrosion, or loose connections

Interpretation: Damaged wiring/connectors likely if physical defects are found
Measure O2 sensor signal voltage with live data or multimeter

Expected: Normal downstream voltage should be stable and not excessively high (>1.0V)

Interpretation: Persistent high readings indicate sensor or circuit fault
Test for short to battery or reference voltages in sensor circuit

Expected: Signal wire should not show 12V or constant high reference

Interpretation: High voltage presence suggests wiring short or PCM issue
Replace the oxygen sensor if confirmed faulty

Expected: Resolved voltage readings within spec

Interpretation: Confirms sensor was defective if code clears after replacement

Repair Solutions

Replace Bank 1 Sensor 2 oxygen sensor medium

Estimated Cost: $100 - $300

Typical aftermarket and OEM sensor costs vary; labor depends on vehicle

Repair or replace damaged wiring/connectors medium

Estimated Cost: $50 - $200

Cost varies by extent of damage and labor time

Repair exhaust leaks before the sensor medium

Estimated Cost: $100 - $400

Depends on location and severity of the exhaust leak

ECM reprogramming or replacement hard

Estimated Cost: $300 - $1200

Only if diagnosis confirms ECU internal fault

Common Misdiagnoses

Avoid these common diagnostic mistakes:

Assuming the oxygen sensor is faulty without checking wiring or connector issues

Frequently Asked Questions

What does the P0138 code mean?

The P0138 code stands for O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). This generally indicates an issue with the Emissions system.

Can P0138 cause engine damage?

You should treat P0138 as a medium issue. Check Engine Light illuminated; possible decreased fuel economy and minor performance issues

How do I know if I have code P0138?

Drivers often report: Check Engine Light illuminated, Reduced fuel economy, Rough idling when this code is present.

Potential causes for OBDII code P0138

The most common causes are: Faulty Bank 1 Sensor 2 oxygen sensor, Wiring or connector issues (short to voltage, corrosion, damage), Exhaust leaks before the sensor.

Repair cost breakdown for P0138

It depends on what needs to be fixed. Repair or replace damaged wiring/connectors repairs are cheaper (~$200), while ECM reprogramming or replacement repairs are on the higher end (~$1200).

Is the P0138 code specific to Mercedes?

The P0138 code is a generic powertrain code, meaning it applies to almost all makes and models, such as Honda, Toyota, VW, Ford, and Chevy.

P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)