Home TroubleshootingOBD Codes P0138: Description, Possible Causes & Fixes For Your O2 Sensor

P0138: Description, Possible Causes & Fixes For Your O2 Sensor

by Kelvin Yates

Car owners live in constant fear of the Check Engine Light coming on because this light can mean anything from a loose electrical wire to a catastrophic problem with the engine. Whenever your motor is suffering from some internal issue, the computer of the vehicle stores error codes. Upon diagnosis, these error codes reveal possible concerns. Like all diagnostic error codes, P0138 is associated with possible motor problems. Know what this DTC stands for and how to solve it.

Code P0138 signals a problem with the oxygen sensor for Bank 1, Sensor 2. The O2 sensor evaluated the vapors emitting from the catalytic converter. You can live in denial and forget this issue even exists when the code pops up on your diagnostic tool, or you can take the sensible route out and do something.

Although these sensors are not mandatory for running the car or safely driving, they may indicate a more severe issue with the engine. Ignore it for too long and it might require rather expensive fixes in the future.

Keep reading to understand how a P0138 affects your car and how to approach this situation.

What Does Code P0138 Mean?

OBD-II Trouble Code P0138 Means: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

Problem Seriousness: Moderate – Prolonged driving with this stored code can result in internal motor damage.

Repair Urgency: Fix this within a month to prevent damage to the internal engine.


Diagnosis: A full diagnosis has to be completed when diagnosing this code. A DTC like this can be triggered by wrong fuel pressure or a bad oxygen sensor.

If your car stores a P0138 code, this translates to the downstream oxygen sensor experiencing an exaggerated voltage reading. The incorrect balance of air and fuel inside your motor causes the high voltage at this sensor. On average, the voltage reading stands at 0.45 volts for the best fuel-to-oxygen ratio.

Code 0138 is displayed when the voltage reading exceeds 1.2. This means there is too much fuel but not adequate oxygen coursing through the engine. We can term this condition “engine running too rich.

P0138 O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

Modern-day vehicles are equipped with at least 2 O2 sensors. Despite both sensors detecting oxygen levels in the exhaust stream, the data they provide is used differently by the PCM. Sensor 1, or the first sensor is called the upstream oxygen sensor. This sensor is the primary unit of feedback in the “closed-loop” gasoline/diesel system and helps the PCM regulate the fuel-air ratio in the vehicle.

Moving onto the second sensor, or sensor 2, is called the downstream oxygen sensor. The job of this one is mainly to monitor the oxygen storage capacity of the catalytic converter.

Many mechanics are unaware that the primary (upstream) O2 sensor can become unreliable on a vehicle, forcing the PCM/ECM to stop depending on it for the signal. When this happens, the downstream O2 sensor will become the primary source of information for the PCM/ECM for fuel trim changes.

Because the ECM/PCM’s algorithm prioritizes the protection of the catalytic converter – the most important and expensive emissions part in the system. A conventional zirconia upstream O2 sensor generates a voltage signal developed to switch back and forth continuously. The reading provides the PCM with an idea of the motor’s fuel-air mixture. A reading over 0.45 volts suggests a rich condition whereas a reading under 0.45 volts suggests a lean state.

Why Does It Happen?

The upstream O2 sensor must be switching actively while the system remains in a “closed-loop,” meaning it’s constantly utilizing the upstream sensor to control fuel. At the same time, if the upstream sensor and catalytic converter are functioning properly, the downstream O2 sensor will drift slightly over 0.5 volts.


By no means should it replicate the operation of the upstream sensor if oxygen is being stored properly by the catalyst?

Code P0138 is stored when the second sensor transmits a voltage signal that is too high for a prolonged period. Too high generally stands for over 1.1 volts – a measure quite high for a conventional zirconia style sensor.

The Bank 1 part of the code suggests the problem is on the engine’s side that holds the number 1 cylinder (applicable for engines with a boxer or “V” configuration). For engines with four sensors and two banks, you have to be able to differentiate between Bank 1 and Bank 1 so you know you are dealing with the correct oxygen sensor.

Bank 1 is where spark plug #1 is located. Make sure to determine which is the first bank to save a lot of effort and time. Oxygen sensor 1/1 is the primary or upstream sensor on Bank 1 (in Ford’s case it is different), and oxygen sensor 1/2 is the second/downstream sensor on Bank 1. Oxygen sensor 2/1 is upstream on Bank 2 whereas oxygen sensor 2/2 is downstream on Bank 2.

As the P0138 code is specifically for sensor 2, the downstream oxygen sensor is what interests us for the time being.

Symptoms Of Code P0138

Your vehicle will display signs of ailment when code P0138 is stored in it. The symptoms are the same for P0138 Nissan, P0138 Toyota, or even P0138 Dodge. The first sign you will notice is an illuminated Check Engine Light. However, soon enough, other symptoms will follow.

You may observe a reduction in vehicle fuel economy. Your vehicle operates rich under these conditions. This essentially means that there is too less oxygen and too much fuel in the motor being burned. Thus, you will have to refill gas more than usual. With the soaring prices of fuel, we all know how big of an inconvenience that can be.

When you are idling or driving, you may get a whiff of a strong smell from the exhaust (making you wish you knew how to get rid of cigarette smell in car or how to get cigarette smell out of car), much stronger than regular exhaust fumes. You may also notice overly rough idling.

If any of these signs show up along with the P0138 code, the issue is not a bad sensor. Check your vehicle at a certified professional’s immediately because something more serious is cooking under the hood.

Possible Causes Of P0138 Code

A worn-out O2 sensor is only one of the many reasons your vehicle might be displaying a P0138 code. This DTC can also show up when any of the associated parts fail. We suggest you have a proper understanding of what triggers a P0138 to save yourself money and time checking and/or fixing the problem. Below is a list of common reasons:

  • Damaged connections or wiring
  • Bad oxygen sensor
  • Problem with the ECM/PCM, such as software update required
  • Engine running too rich due to fuel delivery issues (example: overwhelming fuel pressure)

If the nearby connections are working, the problem could be the oxygen sensor itself. In case that is the sole issue, you will see a P0138 code. A contaminated catalytic converter tricks the system and increases the readings on Bank 1, Sensor 2. Clean this component to resolve this issue.

If none of the above check the boxes and you cannot seem to put your finger on what the exact issue is, inspect the fuel pressure. Extra gas runs through the engine if the fuel pressure is high, affecting the oxygen ratio. An issue with the fuel pressure is a significantly bigger problem than one with the connections or sensor.

P0138 Causes

  • Bad Oxygen Sensor– A bad oxygen sensor can cause a heap of problems for a vehicle. You might be tempted to replace this part right away if the code is stored in your car, but we recommend checking out the connections around it to make sure it is indeed the O2 sensor behind the concerns.
  • Damage to the Terminal– We are talking about the terminal connecting to the oxygen sensor – it is also prone to damage. Make sure it has a supply of voltage to it.
  • Wiring Harness Problems – The wiring traveling to the rear and front O2 sensors is very susceptible to decay. This is because the oxygen sensors are underneath the chassis. There, they are constantly subjected to friction from the road debris and exhaust heat. The downstream O2 sensor is especially vulnerable to this concern. Check out this video on how to detect a short in the wiring harness.
  • Wrong Oxygen Sensor– Replaced the O2 sensor recently but still getting a P0138 code? The replacement may not have been done properly or the replacement part was not correct and/or is faulty.

Is Code P0138 Serious?

You can drive short distances if your vehicle displays a P138 DTC. But, we do not recommend traversing long distances without checking out the underlying reasons for this issue. Operating on a higher fuel-air mixture does not do much harm to your vehicle at first, however, in the long run, the exhaust will be less clean and the fuel efficiency will go down.

Driving a rich engine for prolonged periods can damage your vehicle immensely.

How Does A Mechanic Troubleshoot The P0138 Code?

  • Scans codes before noting freeze frame data. They take that information to clear codes and verify failure.
  • Checks oxygen sensor data to understand if the voltage is moving back and forth too fast (in comparison to the other sensors).
  • Inspects the oxygen sensor harness connections and wirings for corrosions in them.
  • Examines the oxygen sensor for fluid contamination or physical damage.
  • Inspects the sensor for exhaust leaks.
  • Abides by the automaker’s particular instructions for further troubleshooting.

DIY Steps To Diagnose Code P0138

You can fix many DTC at home with the proper guidelines and tools. However, not many people are confident in their mechanical skills and would rather throw money in the professional’s way to get things done.

Diagnosis Requires A Handful Of Specialized Tools

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 an OBD-II scanner tool can offer, but, on regular cars, this should work.

DIY difficulty level: Easy

Tools/parts needed (these are our top picks):

Step 1:

Check all present DTC using an OBD-II scanner tool. Use this helpful tool to make sure that the only code present in your vehicle is P0138. Address other codes if anything shows. Check out this video guide on it.

Step 2:

Check the connections and wiring. Examine all of the connectors and wiring associated with the oxygen sensor.

Step 3:

Inspect O2 sensor voltage. Check the voltage; if it is steadily high (higher than or equal to 0.9 volts) the oxygen sensor might be bad. Check the following before replacing:

If the oxygen sensor is reading overly high voltage with the coolant temperature and fuel pressure being within spec, change the Bank 1 Sensor 2.

Step 4:

Consult a professional. After all, if the car still displays the same code, a more severe problem might be bothering the coolant system of your vehicle’s motor. Bring your car to a certified auto shop to get further diagnostic work done.

Common P0138 Diagnosis Mistakes

Do not leave the diagnostic process halfway when checking the P0138 code. Do not overlook a seemingly simple reason like a damaged or loose connector. Being near the hot exhaust and ground, the plastic connectors and wiring for the O2 sensor are prone to damage.

How To Fix P0138 Code


The effort and cost associated with resolving a P0138 code are based on the underlying reasons of the reading. Typically, these are the possible fixes:

  • Replacing or repairing the connection or wiring to the oxygen sensor for Bank 1 Sensor 2 location
  • Changing the oxygen sensor for Bank 1 Sensor 2
  • Fixing a leaking injector
  • Changing the catalyst preceding the sensor

Start by replacing any deteriorated wires or connections surrounding Bank 1, Sensor 2. In many cases, this step removes the code with little to no cost and effort.

Alternatively, diagnose the O2 sensor itself. The displayed voltage reading should be close to 0.45 volts. Use a multimeter for this step. If it checks over 0.9 volts, the sensor might be damaged and require replacement.

However, before changing the oxygen sensor, make sure that the high voltage reading is not the result of another issue. Ensure both the fuel pressure and coolant temperature are within appropriate ranges. If the fuel pressure and coolant are at their optimum levels, replace the O2 sensor to remove the DTC.

Repair/Replace These Parts To Clear OBD Code P0138

Not all vehicles function the same. P0138 Honda might differ from P0138 Jeep. Repair or replace these components (based on your vehicle) to clear the DTC in question:

  • Oxygen Sensor – Bad O2 sensor can be the answer to why the P0138 code is stored. You can buy an oxygen sensor online and do the replacement yourself or you can reach out to a mechanic.
  • Mass Air Flow sensor – A bad Mass Air Flow sensor can also be the cause of P0138 code displaying.
  • Manifold Absolute Pressure sensor – Replace the Manifold Absolute Pressure sensor if this part has gone bad.
  • Exhaust Parts – Not many car owners pay attention to the exhaust system until something bad happens to it. Remember to check the exhaust system during your regular maintenance schedule.
  • Powertrain Control Module – The PCM, or Powertrain Control Module, has a big role in the vehicle. Naturally, a problem with this will store a code in your vehicle’s computer and it can be code P0138.
  • Engine Control Module – In the brains of your vehicle, the ECM is not one component you consider when thinking of parts to check. A faulty Engine Control Module can trigger lots of problems.
  • Fuel Pump

How Much Does It Cost To Fix Code P0138?

When you bring your vehicle to a mechanic for diagnosis, they are likely, to begin with, an hour of “diagnosis time.” During this time, the mechanic will diagnose the specific issue. Based on the labor rate of the shop, this generally costs anywhere between $75 to $150. Shops generally apply a diagnosis fee to any needed repair if they have to do the repairs.


Remember to ask for an estimate before starting the repair. You can browse local auto shops to find one that is both cheap and effective.

Possible Repair Costs For P0138 Depending On Parts

For the P0138 code, one or more of the repairs stated below may be needed to fix the causes. For each potential fix, the estimated repair cost includes the price of the related parts as well as the labor costs required to perform the repair:

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

Replace O2 Sensor As Precautionary Measure? Here’s Why It’s A Good Idea

While the vast majority of auto shops and motorists change an H02S solely when it illuminated a Check Engine Light, there is a case to consider for changing it as a precautionary measure. Why is that relevant? An O2 sensor with over 100,000 miles on it responds slower than a new part. While this can mean reduced fuel efficiency in the short term, the problem exacerbates when left unattended for long.

Over a long period, a slow oxygen sensor has negative effects on the other components of your vehicle, some of which are quite pricey to repair or replace. When an oxygen sensor cannot respond efficiently, the engine fires with excessive fuel and not much air.

For the most part, excessive unburnt fuel is deposited in the cylinder and intake heads. Slowly, this fuel reaches the crankcase and pollutes the soil, hindering lubrication. Inadequate engine lubrication can decrease the life of your engine drastically.

Left unchecked, lean, or rich condition of an engine can wear out other emissions sensors too, lead to combustion system problems, cause internal engine failure, or destroy the catalytic converter.

At this point, a small repair is mutated into an issue that will set you back a few thousand dollars.

Misleading Oxygen Sensor Codes

When it is not the oxygen sensor, these issues can trigger a P0138 code:

  • Engine vacuum leak creating a lean exhaust state
  • A damaged catalytic converter
    Failure of Mass Airflow Sensor leading to rich exhaust condition
  • Failure of Manifold Air Pressure Sensor leading to rich exhaust condition
  • Ignition misfires creating a rich state
  • Leaky fuel injectors create a rich engine state
  • Exhaust leaks contributing to a lean exhaust state
  • A bad fuel pressure regulator
  • Damaged or burnt connectors and/or wiring
  • A blown fuse

Tips And Tricks

After necessary repairs have been performed, clear the triggered codes from the engine control module and take your vehicle for a test ride. Keep the speed between 30 to 35 miles an hour for around 2 miles. If the oxygen sensor signal drops to a negative voltage or displays an excessively positive one, know that the sensor is still bad.

P0138 Diagnostic Trouble Code: Facts You Need to Know

  1. P0138 is a diagnostic trouble code (DTC) for “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2).”
  2. The code means the O2 sensor for bank 1 sensor 2 fails to have a lower voltage output below 1.2 volts for more than 10 seconds, indicating a lack of oxygen in the exhaust stream.
  3. The Check Engine Light will be illuminated, and the engine may run lean during the testing of the sensor to correct the problem, causing hesitation or misfiring.
  4. The ECM sees the voltage of the O2 sensor for bank 1 sensor 2 above 1.2 volts when it has commanded the fuel to a targeted lean condition on that bank of the engine.
  5. A mechanic needs to diagnose the specific cause for this code to be triggered in your situation.
  6. Common mistakes when diagnosing the P0138 code include misinterpreting sensor operation and not checking for engine leaks or damaged or clogged catalysts (to find out more, check out our guide on what’s in catalytic converters and what are catalytic converters).
  7. To diagnose the P0138 code, a mechanic must scan codes and document freeze frame data and then monitor O2 sensor data, check the O2 sensor wiring and harness connections, check for exhaust leaks, and follow the manufacturer’s specific pinpoint tests.
  8. Repairs that can fix the P0138 code include replacing the O2 sensor, repairing or replacing the wiring or connection to the O2 sensor, replacing the catalyst, and repairing a leaking injector.


Never ignore your exhaust or fuel system problems because you will find yourself in a more expensive mess later. As soon as the Check Engine Light comes on, make it a point to get your vehicle diagnosed. Keep an OBD-II scanner handy for more DTC like P0138. A quick fix right now might save you money, time, and effort in the future.

Frequently Asked Questions (FAQs)

Here are some popular FAQs:

How Do I Fix Code P0138

Look at the list of causes mentioned above. Examine the related connectors and wiring, and check for broken, bent, or damaged components. When you find something, consult a mechanic.

What Is Code P0138

Code P0138 is a diagnostic trigger code associated with an engine condition where the motor runs “too rich.”

Can An Exhaust Leak Cause A P0138 Code

Yes, an exhaust leak can trigger the P0138 code.

What Does It Cost To Replace An O2 Sensor

On average, the cost of replacing an oxygen sensor falls between $155 and $500.

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