Last updated on February 1, 2023
Today’s vehicles are nothing short of engineering marvels. These vehicles maintain efficient operation through the communication of several different modules, each of which monitors a large amount of feedback at a constant rate. Perhaps even more impressive is the fact that these modules have the ability to adjust their outputs on the fly, in response to this feedback.
Although rarely given much thought, a vehicle’s oxygen sensors provide some of the most valuable feedback of all. The feedback obtained from these sensors has a direct relationship to an engine’s fuel trims and the rate at which fuel is consumed. Feedback from the oxygen sensor is also used to determine the efficiency of the catalyst as a whole.
Do you need help with a car problem RIGHT NOW?
Click here to chat online with a verified mechanic who will answer your questions.
Unfortunately, a vehicle’s O2 sensors, while plentiful, are not without their occasional problems. Fortunately, these problems, when they arise, often trigger an OBDII Diagnostic Trouble Code. These codes provide valuable clues that can be used to discover the problem at hand. Of these O2 sensor codes, few tend to be as common as DTC P0139.
Please continue reading to learn more about DTC P0139 and how to troubleshoot these issues, should they arise in the future.
What does the P0139 code mean?
OBD-II Trouble Code P0139 Description
O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
The P0139 diagnostic trouble code indicates that a vehicle’s bank 1/sensor 2 oxygen sensor did not respond in a timely manner, during periods of fuel reduction or fuel cutoff.
This code, as a general rule, relates to a vehicle’s bank 1 post-catalyst O2 sensor. This, in turn, suggests that this particular sensor is incapable of detecting fluctuations in exhaust oxygen content, to the extent that it was designed.
A vehicle’s O2 sensors are designed to maintain preset response rates. When these rates are not met, feedback inaccuracies arise and the corresponding fault code is logged.
In the case of DTC P0139, an engine’s PCM/ECM has determined that the affected 02 sensor has not registered a satisfactory voltage drop (.02V-.03V) within a defined period of time after reducing/cutting made out of fuel. Due to the nature of this code, DTC P0139 is most often set during deceleration.
While a code of this nature may be an isolated fluke, a recurrence or general failure to clear this fault would indicate an ongoing problem. As such, proper diagnostics are required to determine the exact origin of this failure before viable repairs can be made.
While it may be tempting to assume that the oxygen sensor is to blame, that’s not always the case.
See also: Code P0036, Code P0037, Code P0136, Code P0137, Code P0138, Code P2270
Symptoms of Code P0139
The P0139 diagnostic trouble code is often accompanied by a number of secondary symptoms, some of which tend to be more serious in nature than others. Learning to recognize these symptoms can be key when trying to remedy the problem at hand.
The following are the most common symptoms associated with DTC P0139.
Causes of Code P0139
The P0139 diagnostic trouble code can be caused by one of several different underlying conditions, some of which can be relatively difficult to isolate. Understanding the possible causes of this DTC can avoid unnecessary hassle when trying to form a diagnosis.
The following are several of the most common causes of DTC P0139.
Is the P0139 code serious?
While the presence of an active P0139 trouble code by itself is not too alarming, a possible cause of this DTC may be. Sometimes DTC P0139 can be set by the presence of excess fuel in the engine exhaust, especially during periods of deceleration. This raw, unburned fuel can cause significant damage to a vehicle’s catalytic converter in a relatively short time.
On the other hand, the P0139 diagnostic trouble code may simply be the result of a compromised O2 sensor, which in and of itself is not extremely concerning. However, there is little way to discern between the condition and the one described above, without a full diagnosis. For this reason, DTC P0139 should be taken seriously, out of an abundance of caution.
In either case, the root cause of the P0139 diagnostic trouble code should be thoroughly diagnosed and repaired as soon as possible. As mentioned, doing so can save you more expensive repairs in the near future.
If you are not comfortable tackling such repairs on your own, or simply do not have enough time, an appointment with a reputable service center should be scheduled at the first available opportunity.
How to fix the P0139 code
The following steps can be taken to help diagnose your vehicle’s P0139 trouble code. As always, be sure to consult the factory-specific service documentation for your particular model of vehicle before attempting such repairs.
#1 – Check for Additional DTCs
Before beginning the diagnostic process, carefully check for the presence of any additional diagnostic trouble codes. Any such code should be thoroughly diagnosed before proceeding.
2 – Inspect the O2 sensor
Begin the diagnostic process by performing a careful visual inspection of the affected bank 1/sensor 2 oxygen sensor. Check for signs of damage and replace as necessary. Also, all related wiring should be checked for visual defects.
3 – Check for exhaust leaks
Using an application specific fog machine, check for exhaust leaks upstream of the affected O2 sensor. Any leaks that are discovered must be repaired before proceeding.
4 – Check for leaks at the fuel injectors
Next, you will need to make sure that none of your engine’s injectors are bypassing the fuel. This is usually done by monitoring the fuel rail pressure bleed or by removing each injector and checking for leaks.
#5: Verify Sensor Inputs/Outputs
You will now ensure that the affected sensor is receiving all the proper inputs (power/ground) using a digital multimeter. The sensor signal voltage should then be checked, while an attendant revs the engine on and off the throttle.
A noticeable lack of response would be indicative of a sensor failure.
Josh has worked as a full-time mechanic for over 12 years, with 6 ASE (Automotive Service Excellence) certifications to his credit. He started his career as an auto technician at the local Toyota dealership and has since gone on to specialize in trucks and diesel equipment.
