The trouble code P0138 will be set when the PCM reads a high voltage from the O2 sensor (bank, sensor 2) for more than 10 seconds. Oxygen sensor 2 is meant to measure the air and fuel that exits the catalytic converter, letting us know that it’s working properly.
When the voltage reads high, it means there is an excessive amount of fuel leaving the catalytic converter. So when it reads a lower voltage, that means there is more air than usual.
Voltage low (.1V) = excessive air
Voltage high (.9V) = excessive fuel
Normal = about 0.45 volts
Recommended multimeter: AstroAI Digital Multimeter
- Faulty engine coolant temperature sensor
- Fuel pressure is abnormally high
- Bad O2 sensor
- Corroded or exposed wiring to sensor
- Engine running rich
- Strong fumes coming from exhaust
- Poor fuel economy
- Engine runs rich (can be both a cause and a symptom)
- Check engine light will turn on
- Rough idle
How to Fix P0138
It’s very common for someone to replace the oxygen sensor without inspecting the wiring, or finding out if there is another reason the vehicle is running rich. If there is something else going on that is causing the car to run rich, then solve that problem first.
Once you’ve made sure there is nothing else causing the vehicle to run rich, move on to the oxygen sensor inspection. Check the wiring and connection for any damage, including corrosion or exposed wiring.
It’s also easy to check the voltage coming from the O2 sensor. If it’s constantly reading high (at about .9V) then it’s most likely faulty. You can also check the coolant temperature sensor to verify that it’s working properly, as a faulty coolant temperature reading can cause this code to set.
Make sure you’re inspecting the right sensor before you decide to buy a new one. Bank 1 sensors will be on the same side of your engine as the #1 spark plug. Look up the location on your vehicle if you aren’t sure where to look.