P0138 Jeep code is a common OBDII code triggered in Jeeps and other automobiles. Usually, the most common reason for the P0138 jeep code is present is when the voltage at the O2 sensor remains high for an extended period of time.
There are also quite a few other reasons why this code may occur. Before the issue creeps on your engine or other components, read through our troubleshooting guide till the end.
What Does P0138 Jeep Code Mean?
The P0138 is a generic DTC code indicating that the downstream Oxygen sensor (Bank-1, Sensor-2) sends an excessively high voltage reading for an extended period. The PCM recognizes that the voltage in the sensor differs from that specified by the manufacturer.
Usually, a voltage above 0.45 volts signifies a wonderful state, while a reading below 0.45 volts describes a mediocre condition. And too much could mean between 0.69 and 1.2 volts.
Bank mark one of the diagnosis code implies that the issue is on side of the engine, which houses the number one cylinder. So, if we break the P0138 code, we’ll find the following things:
- P – Powertrain Control Module
- 0 – A Generic number specified by SAE standards
- 1 – Fuel and Airflow Metering (Bank 1 oxygen sensor issue)
- 38 – The Specific Issue (Heated downstream oxygen sensor voltage output is 0.69 V or more)
P0138 Jeep Code: What Causes Oxygen Sensor High Voltage?
Even though a P0138 error code is commonly associated with a malfunctioning oxygen sensor, that’s not the only one that can cause this problem. It can also be triggered by an issue with the components that are related to that sensor.
For a thorough diagnosis, you must also be knowledgeable about various additional causes. Below is a list of the possible issues that can set the P0138 code on the jeep.
- Defective downstream oxygen sensor
- Damaged or corroded wiring and connection
- Faulty Powertrain Control Module (PCM)
- Elevated fuel pressure
- Malfunctioning catalytic converter
How To Troubleshoot & Fix P0138 Jeep Code?
The first step in fixing an error code P0138 in your Jeep is determining the root cause. Since there are different possible causes of triggering the P0138 DTC code, the troubleshooting and repair process may vary based on the severity of the issue and the origin of the fault.
For instance, parts like oxygen sensors and PCMs must be swapped out first if they are entirely damaged. This is an extremely delicate task, particularly where reputable parts suppliers are present. Anyway, here is what you do.
1. Replace the Downstream Oxygen Sensor
To begin the diagnosis, first, check the oxygen sensor. As you may remember, the prime reason for the P0138 error code in Jeep is a faulty O2 sensor. Generally, O2 sensors wear out after about 75,000 to 100,000 miles, and their efficiency will also lessen.
However, contamination can cause an O2 sensor to fail much earlier. The O2 sensor’s sensing capability is achieved by producing a small amount of voltage, which is proportional to the oxygen content.
So, when the sensor is faulty, it may sense the wrong Oxygen content amount in the exhaust and result in high voltage. You can test the O2 sensor on your Jeep by first identifying the signal wire on the sensor.
After that, using a multimeter calibrated for reading up to one volt, quickly record the voltages between 200 and 800 millivolts, or .2 and .8 volts. If it shows abnormal reading, the O2 sensor is bad and needs replacement.
2. Check the Wiring Harness and Connection
Next, It’s logical to blame your Jeep’s oxygen sensor first for the P0138 error code. So, in case you find the Oxygen sensor in good condition, check the wiring harness and other connections.
If the wires that connect the PCM to the O2 sensor are damaged or corroded, the transmission of the signal will be obstructed. As a result, the PCM will struggle to communicate properly with the oxygen sensor, affecting fuel efficiency.
Therefore, the PCM mistakenly adjusts the fuel-air mixture, resulting in the P0138 code being triggered. So, look at the wiring and connectors to ensure no damage marks. Besides, a simple multimeter can be utilized to observe if sufficient current is flowing through the wires.
If there are damage marks, the wiring harness and connectors should be replaced. It’s advised that you purchase authentic wire harnesses instead of using cheap alternatives.
3. Replace the Faulty Powertrain Control Module
Assuming that you have found the O2 sensor and wiring are all in good shape, what should be focused on? Yes, it’s the big guy Powertrain Control Module.
The PCM of your jeep depends on electrical inputs to locate faulty sensor circuit states. If PCM understands what is incorrect with the thing, it stores a trouble code in the database and turns on the check engine (CE) warning light.
Now imagine the PCM itself is faulty; what will happen? Clearly, you would not receive the correct info. Newer PCMs are quite reliable. However, water that trickles down from the shell of a bursting radiator or a broken windshield seal can travel into PCM connectors.
This affects the connectors or short circuits of various internal PCM components that often result in wrong readings. So, when the PCM is malfunctioning, you immediately contact a mechanic. While some may suggest repairing, we always recommend a replacement.
4. Inspect the fuel pressure
If the fuel pressure in your Jeep rises quickly, the O2 sensor will detect the change in emissions and send signals that the engine’s fuel mixture is too lean. The O2 sensor will have noted the change and will transmit information to the powertrain control unit.
The PCM transmits signals to maintain the electrical operation of every component constantly. Once it receives the inappropriate voltage signal, it stores the code.
5. Check the Catalytic Converter
After finishing all the troubleshooting mentioned above, if you still notice the error code P0138 on your jeep, maybe the catalytic converter needs your attention. Though rare, a malfunctioning catalytic converter can also trigger the code P0138 in your Jeep.
In this case, you should first verify the integrity of the catalytic converter by examining whether it’s gone bad. To verify this, note whether there are any dents, damaged components, or noticeable cracks in the unit.
However, vacuum or back-pressure testing can reveal whether the exhaust system is clogged. If there is any evidence of inconsistencies with your catalytic converter, you must bring your vehicle to the mechanic for repair.
You still have more queries about the P0138 Jeep code remaining in your mind. The following FAQ section is designed to address those queries.
Can you drive a Jeep with P0138 Code?
Theoretically, you can drive while the error code P0138 is triggered. However, since this represents a moderately serious condition, you should stop driving as soon as you notice it. In fact, driving with this code for a long time might create further damage to your jeep.
How Much Does It Cost to Fix Jeep P0138 Code?
It depends on the root cause of the code. For example, the O2 sensor for Jeep may cost you $25 to $100. If you need to replace it, add the labor cost as well. The cost of fixing the Jeep P0138 code, on average, could be around 100$-$250.
What is the most common Jeep P0138 diagnosis mistake?
Jeep owners’ most common mistake while dealing with code P0138 is replacing the O2 sensor without proper diagnosis. Yes, it’s logical to accuse the O2 sensor; still, sometimes, a simple wiring repair can fix the code.
P0138 Jeep Code signifies an issue that can creep up to a major repair bill. Act soon when the code is set, and take your Jeep to the repair center.
You can still drive while having the code; however, ignoring it for a long time might result in a bigger problem down the road.