P0138 – Meaning, Causes, Symptoms, & Fixes

Code P0138 Definition

O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

What Does P0138 Mean?

Oxygen sensor 2 is the downstream oxygen sensor in relationship to the catalytic converter. It measures the air-fuel ratio coming out of the catalytic converter to ensure the catalytic converter is functioning properly. The catalytic converter works to maintain the stoichiometric air-fuel ratio 14.7:1 while the powertrain control module constantly flip-flops between rich and lean air-fuel mixtures due to the input from the upstream oxygen sensor (sensor 1). Therefore, the downstream oxygen sensor (sensor 2) should produce a steady voltage of approximately 0.45 volts. When trouble code P0138 is set, this indicates that there is high voltage (above 1.2 volts) for more than 10 seconds indicating a lack of oxygen in the exhaust stream in Bank 1.

P0138 Symptoms

  • Check Engine Light
  • Engine runs lean
  • Lower fuel economy
  • Rough idle
  • Strong fumes

P0138 Causes

  • Faulty O2 Sensor
  • Short to battery voltage in O2 sensor signal circuit
  • Corroded wiring

Code P0138 Severity – Moderate   

It is okay to drive a vehicle with P0138 for a short period of time, but driving with this code for an extended period of time can overheat the engine and cause internal engine damage.

Code P0138 Common Diagnosis Mistakes

Replacing the O2 sensor without checking any wires or clogged/damaged catalyst.

P0138 Diagnosis

Difficulty of Diagnosis and Repair – (1-4)

  1. Check around the O2 sensors for visual possible corrosion that might need to be cleaned.
  2. Check for damage to exhaust pipes and damage to the catalytic converter.
  3. Check live data from all 4 terminals of O2 sensors. Make sure your key is on and your engine is off when checking this. You are checking for ground and signal reference. As long as you are getting ground and .3 volts of signal reference.
  4. If these steps do not clear your Check Engine Light, you may need to visit a mechanic because there may be internal electrical issues.

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Comments 16

Dale R Stein

September 6, 2018

I’ve replaced my o2 sensors and am still having problems. Now I know to check the entire system.

Ernest C Herzberg

October 13, 2018

I have fixed the problem thanks


November 4, 2018



January 17, 2019

How did you fix the problem if I may ask


April 4, 2019

What you do to fix the problem. Please share..

James Reinheimer

October 13, 2018

Had cat replaced 30 days ago,no ck engine light ever came on. Now ck engine light came on bank one sensor 2. I replaced that o2 sensor. Now light came back on.same code

Fred Bracaglia

January 10, 2019

Any update on fix .I have 2010 mazda3 and getting same code.


January 25, 2019

Check or replace the pcv valve.


July 15, 2019

Fred, I have same car, year and same problem. did you fix yours?

Antonio P

November 4, 2018

The Code has constantly been coming on I’ve replace the O2 sensor four times now and had a smoke test done and checked the wiring and everything came back negative still don’t know what the problem is. Please help

Adam Young

November 27, 2018

Having same issue with my 2012 rav 4. Replaced the O2 sensor downstream, check engine light came back 1 day later. Was recommended that I replace the upstream O2 sensor because it could be causing a false reading on the 2nd O2 sensor. That may be my next step, will post update.

Steve Levy

January 21, 2019

Did you see an i.provement?

Byron Freeman

December 19, 2018

How much should this repair cost? Thanks.


February 24, 2019

I purchased an O2 isolator- casot about 5$- it screws into the O2 port then you screw the O2 sensor into it- Cleared the codes and the engine ran smoother than ever and no more codes or CEL.
I had replaced the O2 sensor but it did not change until the isolator was installed.
Amazon has them.


April 30, 2019

Tom, Feb 24, 2019/ Seems a good place to ‘start’, using the O2 isolator. They are on Amazon. Thanks for the tip.


July 17, 2019

Can you provide the exact part I should order? Do you have an install guide i could follow? Thanks!

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