The P0138 is an OBD-II trouble code that stands for “02 Sensor Circuit High Voltage (Bank 1, Sensor 2)” and is triggered when your vehicle’s primary computer, also known as the powertrain control module (PCM), detects that the voltage signal from the rear O2 sensor is too high for a given period.
In this article, I will be talking about the P0138 trouble code, its symptoms, causes, and how to fix it.
What Does The P0138 Code Mean?
When your vehicle stores a P0138 trouble code, it means that the downstream oxygen sensor detected an abnormal voltage. When the voltage at this sensor is too high, the engine doesn’t get the right amount of air or fuel.
The O2 sensor also referred to as the Oxygen sensor is designed to measure the amount of oxygen in the exhaust gases coming out of the combustion chamber. When the signal from the O2 circuit is too high, the sensor is unable to measure the right amount of exhaust gases and the vehicle’s computer will trigger a code.
For the optimal oxygen-to-fuel ratio, the standard voltage reading of the O2 Sensor is around 0.45 volts. Code P0138 is displayed when the sensor voltage exceeds 1.2. Your engine has too much fuel and not enough oxygen because the voltage is high.
Which Models Are Affected By The P0138 Code
| Make | Affected By P0138 Code |
|---|---|
| Toyota | Yes |
| Nissan | Yes |
| Chevrolet | Yes |
| BMW | Yes |
| Audi | Yes |
| Buick | Yes |
| Mercedes | Yes |
| GMC | Yes |
| Suzuki | Yes |
| Tata | Yes |
| Lexus | Yes |
| Mazda | Yes |
| Mitsubishi | Yes |
| Kia | Yes |
| Jeep | Yes |
| Fiat | Yes |
| Honda | Yes |
| Hyundai | Yes |
| Opel | Yes |
| Ford | Yes |
| Fiat | Yes |
| Peugeot | Yes |
What Are The Possible Causes of the P0138 Code?
The P0138 is caused when the vehicle’s computer detects higher voltage output from O2 Sensor bank 1 sensor 2.
The P0138 trouble code can be caused by a variety of underlying issues, some of which are far more common than others. Understanding these potential causes can help you repair your car more quickly.
Here are the most possible causes of the P0138 trouble code:
- Faulty oxygen sensor
- In the O2 sensor signal circuit, there is a short to the battery voltage.
- Damaged or corroded wiring
- Fuel pressure is too high
- Temperature sensor for the engine coolant
What Are The Common Symptoms of The P0138 Code?
There are several additional symptoms that often accompany the start of the P0138 trouble code. However, these symptoms may vary from case to case.
Here are the most common symptoms of the P0138 trouble code:
- Check engine light: The most noticeable symptom of a P0138 trouble code is the illumination of the check engine light. This light is on the dashboard of the car and can be on all the time or flash.
- Reduce engine performance: When the engine runs too rich, the engine’s performance suffers, and the vehicle may not accelerate as quickly as it should.
- Poor fuel economy: If the oxygen sensor finds that the mixture of air and fuel is too rich, it can lead to poor fuel economy. This means that to go the same distance, the car may need more gas than usual.
- Rough idling: If the air-fuel mixture is too rich, the engine will idle rough or stall.
- Heavy exhaust fumes: You can smell the exhaust gas when you are driving or even when the car is not moving. This is because a broken O2 sensor can cause the ratio of air to fuel to be wrong, which makes the fuel not burn completely. These emissions will have a strong odor as well.
It is important to keep in mind that the symptoms of the P0138 trouble code can vary depending on the make and model of the car and how bad the problem is.
(See also: Can a Bad O2 Sensor Cause a Misfire? (Answered!))
Is It Safe To Drive With a P0138 Code?
Yes, it is safe to drive with a P0138 trouble code, but only for short distances. Therefore, you should not wait too long before investigating the root cause of the code.
Using your car with a higher fuel-to-oxygen ratio does not cause immediate damage. But it will decrease your fuel economy and make your exhaust less clean. The long-term, wealthy operation will eventually damage your vehicle.
How To Diagnose The P0138 Code
If you want to diagnose the P0138 trouble code easily, follow the steps below.
- To verify the issue, use a scan tool to check the codes, document the freeze frame data, and clear the codes.
- Examine the O2 sensor for damaged wiring, a loose connector, or corrosion in the wiring harness.
- Compare the sensor voltage data of sensors 1 and 2 and test the O2 sensor for low or high voltage. Sensor voltage should be nearly identical, but sensor 1 should have a higher oxygen level reading.
- Examine the oxygen sensor for physical damage or fluid contamination caused by engine leaks.
- Check the engine coolant temperature sensor for functionality.
- Check for possible high fuel pressure and test the fuel pressure regulator to see if it meets manufacturer specifications (a low air-fuel ratio).
- Check for an exhaust leak before the sensor and a loose gas cap. Also, check to see if the catalytic converter is clogged or damaged.
- Other pinpoint tests for Bank 1 and Sensor 2 issues specified in the manufacturer’s manual should be performed.
(See also: How To Clean O2 Sensor Without Removing It)
Most Common Mistakes When Diagnosing The P0138 Code
To diagnose a P0138, compare the sensor voltage between Bank 1, Sensor 1, and Sensor 2: Both sensors should be operating at nearly identical voltages, but sensor 2 should show a lower O2 reading because the catalytic converter burns off excess fuel and O2. If you don’t see the O2 differential, there could be another cause.
Many do-it-yourselfers and some technicians also fail to inspect Sensor 2 for oil or coolant contaminants from any engine leaks, which can result in low O2 readings even if the sensor is working properly. A damaged catalytic converter is another potential issue that is easy to overlook. It can cause erratic sensor readings and a P0138 if it becomes clogged with carbon or leaks air.
How Much Does It Cost To Repair P0138 Code
| Make | P0138 Code | Repair Cost |
|---|---|---|
| Toyota | Yes | $100 – $1500 |
| Nissan | Yes | $100 – $1500 |
| Chevrolet | Yes | $99 – $1470 |
| BMW | Yes | $100 – $1500 |
| Audi | Yes | $100 – $1500 |
| Buick | Yes | $70 – $1300 |
| Mercedes | Yes | $100 – $1600 |
| GMC | Yes | $100 – $1500 |
| Suzuki | Yes | $100 – $1500 |
| Tata | Yes | $60 – $1480 |
| Lexus | Yes | $100 – $1500 |
| Mazda | Yes | $100 – $1500 |
| Mitsubishi | Yes | $100 – $1500 |
| Kia | Yes | $85 – $1485 |
| Jeep | Yes | $100 – $1500 |
| Fiat | Yes | $100 – $1500 |
| Honda | Yes | $100 – $1500 |
| Hyundai | Yes | $100 – $1500 |
| Opel | Yes | $72 – $1375 |
| Ford | Yes | $100 – $1500 |
| Fiat | Yes | $80 – $1400 |
| Peugeot | Yes | $100 – $1500 |
What Repairs Can Fix The P0138 Code?
Repairs that can fix the P0138 trouble code include the following:
- If the wiring harness or electrical connections are damaged, corroded, or loose, repair the circuits or replace the connectors.
- In the event of a short due to high battery voltage, repair the O2 sensor signal circuit.
- If the engine coolant temperature sensor and fuel pressure regulator are faulty, they must be replaced.
- If the downstream oxygen sensor (rear sensor) on Bank 1 is faulty, it must be replaced.
- Replacing a faulty fuel injector or catalytic converter.
- Repairing any existing exhaust leaks.
How To Fix The P0138 Code
Here are some steps that you should take to fix the P0138 trouble code:
- Replace Sensor 2 with a new O2 sensor.
- Repair or replace the O2 sensor wiring or connectors for Bank 1 and Sensor 2.
- Replace the catalytic converter in front of the sensor.
- Repair the leaking injector.
In Conclusion
This problem, like many fuel and exhaust system problems, can go from being easy to fix to cost a lot to fix.
If your check engine light comes on, read the trouble codes on your own OBD-II scanner or take it to a mechanic right away. A quick fix at the start can save you a lot of time, money, and headaches later on.