P0139 is a common OBDII code that occurs in many vehicles, including the Jeep Liberty. The code is triggered when there is an issue with the response time from the oxygen (O2) sensor when switching voltage levels.
The Oxygen sensors are responsible for helping the ECM meter oxygen levels in the exhaust, and helping to to create the ideal air/fuel mixture.
When they take too long to respond, the ECM can no longer depend on that information to manage the mixture.
There are also some less likely problems that can cause P0139. We’ll go over those below.
Bank 1 = Side of the engine with cylinder 1 in the firing order
Sensor 2 = Aft of the Catalytic Converter
Jeep Liberty P0139 Symptoms
Typically, the only symptom of P0139 is going to be the service engine soon light. In some cases, there can be other symptoms as well. These symptoms include:
- Increased emissions
- Smelly exhaust
- Service light on
- Pinging
Often, the vehicle with P0139 will not pass emissions due to the higher levels of pollutants in the exhaust. That assumes that the sensor is functioning properly and isn’t the reason why the code was thrown.
A poor air fuel mixture going through the catalytic converter can cause damage to it eventually.
Here’s a helpful video from YouTube:
Jeep Liberty P0139 Causes
Here are the most common issues that cause the P0139 code in the Jeep Liberty. 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 P0139 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 P0139 after having recently replaced your Oxygen sensor, it may be that the replacement is wrong or bad.
Liberty P0139 Diagnosis
It is possible to diagnose the P0139 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 Liberty’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 P0139 trouble code.
Conclusion
Good luck finding whatever caused P0139 in your Jeep Liberty. If you have anything that you would like to add, please feel free to leave a comment below.