Air & Fuel Mixture ControlOBD2 Codes

P0138 – Meaning, Causes, Symptoms, & Fixes

Reading Time: 2 minutes

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 amount of air and fuel coming out of the catalytic converter to ensure the catalytic converter is functioning properly. If the voltage is high (approx. .9V) there is an excess of fuel in the mixture. If the voltage is low (approx. .1V) then there is an excess of air in the mixture. 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 a high voltage (steadily above .9 volts) for more than 10 seconds indicating a lack of oxygen in the exhaust stream and an abundance of fuel at sensor 2 on the bank 1 of the engine.

What Are the Symptoms of Code P0138?

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

What Is the Cause of Code P0138?

  • Faulty O2 Sensor
  • Short to battery voltage in O2 sensor signal circuit
  • Corroded wiring
  • Fuel pressure too high
  • Engine coolant temperature sensor

How Serious Is Code P0138? – 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 damage the catalytic converter(s).

Code P0138 Common Diagnosis Mistakes

Replacing the O2 sensor without checking to see if the vehicle is running rich due to another problem.

Tools Needed to Diagnose Code P0138:

How to Diagnose and Repair Code P0138:

Difficulty of Diagnosis and Repair – (2 out of 5)

  1. Check the wiring at O2 sensor for damage; if damaged, repair or replace affected harness.
  2. Check voltage at the O2 sensor; if the voltage is steadily high (.9V or higher) the O2 sensor may be faulty. Before replacing, check the following:
    1. Verify coolant temperature is reading properly (consult vehicle manual for specification)
    2. Verify fuel pressure is within spec.
  3. If the O2 sensor is reading high voltage and the fuel pressure and coolant temperature are within spec, replace the bank 1 downstream O2 sensor.

Estimated Cost of Repair

For error code P0138, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.

  • Oxygen sensor $200-$300
  • Fuel pressure regulator $200-$400
  • Engine coolant temperature sensor $150-$200

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24 Comments

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

    1. It could be a leak in the Exhaust pipe thats why problem is

  2. I have fixed the problem thanks

    1. How?

    2. How did you fix the problem if I may ask

      1. How did you fix the problem

        1. I replace both 02 sencent and the code return the same day
          P2271&p0138

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

      1. I went ahead and replaced both of the 02 sensors, now after 2 days the code po138 came on. It wasn’t on before does this mean the 02 sensor is bad?

  3. 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

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

      1. Check or replace the pcv valve.

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

  4. 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

    1. Check for exhaust leak too just to be sure had the same fault codes on my nissan checked exhaust and had a medium leak at the flange before bank 2 sensor

  5. 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.

    1. Did you see an i.provement?

    2. Gas cap replaced and engine light out on a PO 138 ,,

  6. How much should this repair cost? Thanks.

  7. 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.

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

      1. You can get the isolators you guys are talking about from oriellys auto parts except it’s called a spark plug fouler and you don’t have to wait just drill it out and you have an isolator

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

  8. I have this code P0138,but the check engine light isn’t on and my jeep won’t turn over,but I can roll start it

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