Air & Fuel Mixture ControlOBD2 Codes

P0138 – Meaning, Causes, Symptoms, & Fixes

What Does Code P0138 Mean?

  • P0138 definition: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
  • Issue Severity: MODERATE – Extended driving with this code can cause internal engine damage.
  • Repair Urgency:  Get this code fixed withing the next month to prevent internal engine damage. 
  • Diagnosis: It is important to complete the entire diagnostic process when diagnosing the P0138 code. This DTC can be triggered by a faulty oxygen sensor or incorrect fuel pressure.

Find a Certified Shop

We've partnered with RepairPal to recommend trustworthy shops in your area. Enter your details to see certified shops near you that offer upfront estimates, guaranteed fair pricing, and a minimum 12-month, 12,000-mile warranty.

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.

P0138 Causes

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

P0138 Symptoms

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

How Do I Fix Code P0138?

With a P0138 code, the first step is to get it properly diagnosed to figure out what is causing the malfunction in the oxygen sensor voltage. Using the live data function of the FIXD sensor and app allows you to read and analyze inputs from the oxygen sensor to properly diagnose a P0138 code. If you’re not comfortable diagnosing this issue further at home, we recommend finding a RepairPal-certified shop to pinpoint the problem and give an accurate estimate for repairs. 

These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.

> Find a RepairPal Certified Shop Near You

How Much Does It Cost to Fix Code P0138?

If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0138 code.

Right shop, right price

We've partnered with RepairPal to recommend certified and trustworthy shops in your local area. 

  • Upfront cost estimates
  • Minimum 12-month, 12,000-mile warranty
  • Fair price guarantee

Possible Repair Costs for P0138

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

DIY Steps to Diagnose Code P0138

If you’d like to try to fix code P0138 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Diagnosis can require some specialized equipment beyond what the FIXD Sensor can provide, but, on most vehicles, this is still a beginner-level diagnosis and repair for DIYers. 

DIY difficulty level: Easy

This repair is easy for beginner DIYers to attempt.

Tools/parts needed (our top picks from Amazon):


Use FIXD to scan your vehicle to verify P0138 is the only code present. If other codes are present, they must be addressed first.


Inspect all of the wiring and connectors at the oxygen sensor.


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.

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.


If at this point the vehicle is still setting the same code, you may have a more serious problem with your car’s engine coolant system, and you should bring the vehicle to a certified shop to have further diagnostic work performed.  

Common P0138 diagnosis mistakes

It is important to complete the entire diagnostic process when diagnosing P0138. Don’t overlook an easy cause such as a loose or damaged connector. Being so close to the ground and the hot exhaust, the wiring and plastic connectors for the oxygen sensor are susceptible to damage.

Still Need Help Fixing Code P0138?

If you’ve followed the steps above and are still experiencing issues with the coolant temp circuit and code P0138, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price.


Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a ’91 GMC Syclone. #GirlDad #SaveTheManuals

Jeffrey N. Ross
Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a '91 GMC Syclone. #GirlDad #SaveTheManuals

You may also like


  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

      1. My muffler is not welded so my code comes up, so its a leak in the Exhaust for shore,.

  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

    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

    2. Check your fuses and if the fuses are good replace coolant temperature sensor

      1. What fuses should I check?

  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

  9. Check your fuses and if the fuses are good replace coolant temperature sensor

    1. Make sure you replace the correct sensor. There are 2 of them. One for the gauge/idiot light on the dash, the other is for engine controller. If the computer ‘sees’ the engine is cold, even though it may be normal operating temperature, it’ll add fuel, Just like choking a carburetor in cold weather.

  10. I have a 2006 dodge stratus and i got the same code p0138, so I changed it 2x because I thought it was a faulty 02 sensor, and it keeps throwing the same code, and it still is, what do I do?

    1. Did you ever find out the problem. I have a 04 dodge stratus and can’t figure it out.

Leave a reply

Your email address will not be published.