P0138 Code: What it means and how to fix it

Learn what a P0138 code means, the most common causes, and the diagnostic procedure

When your check engine light comes on, it’s never a welcome sight. One of the common trouble codes I’ve encountered is P0138, and if you’re dealing with it, you need to understand what it means, why it’s happening, and how to fix it before it leads to bigger problems. This code is directly related to the oxygen sensor in your vehicle, and diagnosing it correctly is crucial for maintaining your engine’s efficiency and emissions control.

The 2-second answer

If you want to cut to the chase, a P0138 is usually caused by:

• Bad downstream O2 sensor or wiring harness issue
• Vacuum leak or exhaust leak.
• Leaking fuel injectors or high fuel pressure.
• Bad catalytic converter (lost its ability to store oxygen)

What Does the P0138 Trouble Code Mean?

A P0138 Code is a generic trouble code defined as an O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). Bank 1 is the part of the engine that houses cylinder number one. Sensor 2 is located after the catalytic converter. The sensor is used to check the oxygen storage capacity of the converter.

The P0138 trouble code indicates that the oxygen sensor in Bank 1, Sensor 2, is reporting a voltage that is too high for an extended period. This sensor is located downstream of the catalytic converter and monitors how efficiently the converter is working by measuring the oxygen levels in the exhaust gases. High voltage from this sensor usually indicates that the air-fuel mixture is too rich, meaning there’s too much fuel and not enough air in the exhaust gases. In some rare cases, the cause of the P0138 code can be a faulty upstream oxygen sensor (* see this explanation of how that can happen)

Symptoms associated with a P0138 code

• Poor MPG—The engine running rich because it’s getting too much fuel or not enough air.
• Lack of acceleration
• Rough Idle due to an improper air/fuel balance. You may experience a rough idle, misfires, or even stalling.
• Exhaust Smell caused by the rich mixture

What Causes the P0138 Trouble Code?

Several issues can cause a P0138 trouble code, and understanding them is the first step toward a proper fix.

• Faulty Downstream Oxygen Sensor— If the oxygen sensor in Bank 1, Sensor 2 has failed, it can send incorrect voltage readings to the ECM, triggering the P0138 trouble code.
• Wiring or Connector Issues— Damaged, corroded, or shorted wiring can cause a constant high voltage reading. Water intrusion, road salt, and heat exposure can degrade the sensor’s wiring.
• Rich Running Condition— An overly rich air-fuel mixture (too much fuel, not enough air) can cause high oxygen sensor readings. This could be due to a failing fuel injector, a stuck open purge valve, or excessive fuel pressure.
• Bad Catalytic Converter— If the catalytic converter is failing or clogged, it can affect exhaust flow and cause abnormal oxygen sensor readings.
• Malfunctioning PCM (Powertrain Control Module)— While rare, a software issue or internal failure in the PCM could incorrectly interpret the oxygen sensor data, causing a false P0138 trouble code.

What to check to diagnose the code

Scan Long Term Fuel Trims

Use the live data on a scan tool and check the long term fuel trim values. A LTFT above 10% is an indication of an air/fuel mixture problem. In other words, the ECM is adding 10% or more fuel. That’s not normal. The cause can be high fuel pressure, leaking fuel injectors, a vacuum or exhaust leak that’s causing the ECM to add fuel to compensate.

If you have high LTFT, fix the problem before moving on to the P0138 code.

Use a fuel pressure gauge to check fuel pressure and volume.

Check Voltages On The Downstream Oxygen Sensor

Using a multimeter with the engine off but the ignition set to the RUN positions, check for reference voltage, battery voltage for the heater circuit, and both ground connections in the oxygen sensor connector of the downstream oxygen sensor. If the voltages and ground are good, chances are the sensor is bad and not the catalytic converter. Replace the downstream oxygen sensor.

If you don’t see the correct voltages or ground, check the wiring to find and fix the fault.

*In some cases, if the upstream O2 sensor signal becomes so unreliable that ECM/PCM can’t rely on the data it’s getting, it may use the data from the downstream O2 sensor. Normally, the downstream sensor is only used to monitor oxygen storage capacity in the catalytic converter. However, it can be used by the ECM/PCM to monitor fuel trim adjustments. In most cases, this scenario would set a P0132 code to indicate that the upstream sensor has failed.

©, 2024 Rick Muscoplat