P0139 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
Let the exhaust cool fully before inspecting the rear sensor and surrounding wiring
2
Free - no tools
Check for exhaust leaks near the sensor bung or the downstream pipe
3
Basic tool needed
Inspect the connector and harness for heat damage or loose fit
4
Basic tool needed
If scan data is available, compare the downstream signal speed with the upstream sensor and engine behavior
5
Basic tool needed
If the engine has catalyst or mixture codes, treat them as part of the same diagnosis

If the code returns

-If the exhaust is sealed and the signal is still slow, the sensor or wiring becomes more likely.
-If the code returns after a sensor swap, revisit the exhaust side and catalyst pattern before replacing more parts.
-If the signal only looks slow because of another engine-running issue, fix that first.

Background

What this code means

P0139 is a generic OBD-II oxygen-sensor code for bank 1 sensor 2.

A slow downstream response can come from a tired sensor, exhaust-side leak, or catalyst behavior that makes the signal sluggish.

Catalyst-monitor issues or slow switching on live data can show up even when the vehicle still drives normally.

Diagnosis

Common causes

Most common

Aging downstream O2 sensor

The sensor can still work but switch too slowly.

Common

Exhaust leak near sensor

Fresh air can distort the switching pattern.

Common

Wiring or connector issue

Heat damage or poor pin fit can slow the response.

Possible

Catalyst-related behavior

The catalyst can affect the downstream sensor's apparent response speed.

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 proves slow after exhaust checks.

Exhaust gasket or leak repair$20-$200Worth checking if a leak is present near the sensor.

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

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). P0139 was expanded around common slow downstream oxygen-sensor faults, including sensor aging, exhaust leaks, and wiring issues.

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

P0139 - P0139 Usually Means the Bank 1 Sensor 2 Oxygen Sensor Is Responding Too Slowly