P0138 Code Meaning and Next Steps

First checks take 10 to 15 minutes for the first checks. No special tools are usually needed for the first checks.

Can you keep driving?

Stop driving if any of these apply

!The engine is misfiring, stalling, or the check-engine light starts flashing.
!There is a strong exhaust smell or a drivability change that suggests a bigger fault than the sensor alone.

If the light is steady and the vehicle still drives normally: Often yes for a short time, but it should not be ignored.

What to check first

Step-by-step checks

1
Free - no tools
Check the downstream sensor connector and harness for heat damage or looseness
2
Free - no tools
Look for exhaust leaks or recent exhaust work near the rear sensor
3
Basic tool needed
Compare downstream and upstream readings if live data is available
4
Basic tool needed
If the engine has other fuel-trim or catalyst codes, treat them as part of the same diagnosis
5
Basic tool needed
If the code appeared after a repair, verify the correct sensor and connector were installed

If the code returns

-If the exhaust is sealed and the signal still runs high, the sensor or wiring is a stronger suspect.
-If the code returns after a sensor swap, revisit the exhaust side before replacing more parts.
-If catalyst behavior seems abnormal, do not ignore that pattern while chasing the sensor.

Background

What this code means

P0138 is a generic OBD-II oxygen-sensor code for bank 1 sensor 2.

A high downstream signal can come from the sensor, its wiring, or a catalyst or exhaust condition that keeps the reading elevated.

Catalyst monitoring may look wrong even when the vehicle still drives normally.

Diagnosis

Common causes

Most common

Failed downstream O2 sensor

The sensor can bias high and look stuck.

Common

Wiring or connector issue

A heat-damaged harness can distort the signal.

Common

Exhaust or catalyst issue

A larger exhaust-side issue can influence the downstream reading.

Possible

Incorrect part or installation

The wrong sensor or poor fit can create the same result.

Avoid these mistakes

What not to do

xDo not replace oxygen sensors first if there is an obvious exhaust leak or mixture problem.
xDo not ignore rough running just because the code names a sensor.

Parts

Parts that may need replacing

PartTypical costNotes

Downstream oxygen sensor$50-$180Relevant when the sensor is proven biased high.

Exhaust gasket or leak repair$20-$200Worth checking if the exhaust side is compromised.

Connector pigtail repair$15-$90Relevant if the connector is damaged or corroded.

Related OBD codes

P0136

P0136 usually means the bank 1 sensor 2 oxygen sensor circuit is not behaving correctly.

P0137

P0137 usually means the bank 1 sensor 2 oxygen sensor is reading too low.

P0420

P0420 usually means the catalytic converter efficiency is below the threshold the ECU expects.

Source notes

Generic OBD-II (SAE J1979 / ISO 15031-5). P0138 was expanded around common high bank 1 sensor 2 signal faults, including sensor bias, wiring issues, and exhaust-side problems.

This guide is written as a generic multi-make reference, so bulletin history, sensor locations, and repair order can still change by manufacturer and engine family.

This is generic OBD-II guidance and should not override vehicle-specific service information. Exact diagnosis and repair steps vary by make, engine family, and model year.

Last reviewed: 2026-04-10

Reference: Open reference

P0138 - P0138 Usually Means the Bank 1 Sensor 2 Oxygen Sensor Is Reading Too High