P0138 – Meaning, Causes, Symptoms, & Fixes
Code P0138 Definition
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
What Does P0138 Mean?
Oxygen sensor 2 is the downstream oxygen sensor in relationship to the catalytic converter. It measures the amount of air and fuel coming out of the catalytic converter to ensure the catalytic converter is functioning properly. If the voltage is high (approx. .9V) there is an excess of fuel in the mixture. If the voltage is low (approx. .1V) then there is an excess of air in the mixture. Therefore, the downstream oxygen sensor (sensor 2) should produce a steady voltage of approximately 0.45 volts. When trouble code P0138 is set, this indicates that there is a high voltage (steadily above .9 volts) for more than 10 seconds indicating a lack of oxygen in the exhaust stream and an abundance of fuel at sensor 2 on the bank 1 of the engine.
What Are the Symptoms of Code P0138?
- Check Engine Light
- Engine runs rich
- Lower fuel economy
- Rough idle
- Strong fumes
What Is the Cause of Code P0138?
- Faulty O2 Sensor
- Short to battery voltage in O2 sensor signal circuit
- Corroded wiring
- Fuel pressure too high
- Engine coolant temperature sensor
How Serious Is Code P0138? – Moderate
It is okay to drive a vehicle with P0138 for a short period of time, but driving with this code for an extended period of time can damage the catalytic converter(s).
Code P0138 Common Diagnosis Mistakes
Replacing the O2 sensor without checking to see if the vehicle is running rich due to another problem.
Tools Needed to Diagnose Code P0138:
How to Diagnose and Repair Code P0138:
Difficulty of Diagnosis and Repair – (2 out of 5)
- Check the wiring at O2 sensor for damage; if damaged, repair or replace affected harness.
- Check voltage at the O2 sensor; if the voltage is steadily high (.9V or higher) the O2 sensor may be faulty. Before replacing, check the following:
- Verify coolant temperature is reading properly (consult vehicle manual for specification)
- Verify fuel pressure is within spec.
- If the O2 sensor is reading high voltage and the fuel pressure and coolant temperature are within spec, replace the bank 1 downstream O2 sensor.
Estimated Cost of Repair
For error code P0138, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.
- Oxygen sensor $200-$300
- Fuel pressure regulator $200-$400
- Engine coolant temperature sensor $150-$200
P0138 is a common OBDII code that occurs in many vehicles, including theChevy Equinox. The code is triggered when:
- The voltage at the sensor remains above threshold for 20 seconds or more
- Voltage to the sensor is to high (wiring problem, think short)
- Bad O2 sensor itself
There are also some less likely problems that can cause P0138. We’ll go over those below.
Here’s a pretty helpful video from AutoEclinic on diagnosing the code:
Chevy Equinox P0138 Symptoms
Typically, the only symptom of P0138 is going to be the service engine soon light. In some cases, there can be other symptoms as well. These symptoms include:
- Rough running engine
- Smelly exhaust
- Service light on
- Poor MPG
Most of the time, it’s not a breakdown risk. But, like anything that causes the service engine soon light to come on it is a good idea to have it fixed as soon as possible. A poor air fuel mixture going through the catalytic converter
Chevy Equinox P0138 Causes
Here are the most common issues that cause the P0138 code in the Chevy Equinox. They are presented in order (somewhat) of most to least likely to cause the problem.
- Bad Oxygen Sensor– The O2 sensor itself is one of the most likely reasons that P0138 is triggering your service engine soon light. Although it is a temptation to replace it right away, it wouldn’t hurt to check out the wiring harness around it beforehand. See directly below. They are relatively affordable at Autozone or Amazon.
- Wiring Harness Issues– The wiring that goes to the front and rear oxygen sensors is very susceptible to damage. This is due to the fact that O2 sensors are under the chassis where they are subject to punishment from road debris, and they are subjected to a lot of exhaust heat. The downstream sensor is especially susceptible to this problem Here’s how to find a short in the wiring harness (Youtube).
- Damage to the Terminal– The terminal that the oxygen sensor connects to (the plug) is also relatively susceptible to damage. You’ll want to make sure that there is voltage going to it. There’s more on that directly below.
- Wrong O2 Sensor– If you are getting P0138 after having recently replaced your Oxygen sensor, it may be that the replacement is wrong or bad.
Equinox P0138 Diagnosis
It is possible to diagnose the P0138 with a multimeter. If you use the multimeter, you’ll know whether or not the voltage coming to and from the oxygen sensor is within spec, this can help you determine if it’s your Equinox’s O2 sensor, or wiring harness that’s the issue.
If you don’t have access to a multimeter, it certainly wouldn’t hurt to check the wiring to between the fuse box and the Oxygen sensor first. If it ends up being a wiring issue, it’ll save you money over buying an unnecessary O2 sensor.
You can also use a voltage test light to determine whether or not the oxygen sensor is getting any voltage at all (they’re about $5 at Walmart).
If it’s not getting any voltage, attach the test light to the harness plug and shake the wiring a little bit. If the light comes on, even for a second, you know that you have a short circuit causing the P0138 trouble code.
Good luck finding whatever caused P0138 in your Chevy Equinox. If you have anything that you would like to add, please feel free to leave a comment below.
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Chevy equinox p0138
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