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P0138 Code — What it means and how to fix it

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

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 code means the Engine Control Module (ECM) has detected a high voltage signal (over 999 mV) from the downstream sensor for more than 10 seconds. That indicates that either the catalytic converter or the oxygen sensor is not operating properly and the exhaust has very little oxygen. In most cases, the cause is the oxygen sensor.

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

Engine Running Rich that’s caused by too much fuel or not enough air.
Reduced MPG
Lack of acceleration
Rough Idle due to an improper air/fuel balance. You may experience a rough idle, misfires ore even stalling.
Exhaust Fumes caused by the rich mixture

What to check to diagnose the code

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.

Using a scan tool, check the long-term fuel trims to see if they’re above 10%. If so, find out why the fuel trim reading is so high. Check for exhaust or vacuum leaks.

If the P0138 code returns and the long-term fuel is in the negative territory, check for leaking fuel injectors by testing fuel pressure for leakdown.

*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 in this case to monitor fuel trim adjustments. In most cases, this scenario would set a P0132 code to indicate that the upstream sensor has failed.

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