P0138 is a common OBDII code that occurs in many vehicles, including theFord Freestyle. 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:
Ford Freestyle 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
Ford Freestyle P0138 Causes
Here are the most common issues that cause the P0138 code in the Ford Freestyle. 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.
Freestyle 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 Freestyle’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.
Conclusion
Good luck finding whatever caused P0138 in your Ford Freestyle. If you have anything that you would like to add, please feel free to leave a comment below.