The P0139 is a generic code which would indicate that the O2 sensor for bank 1 sensor 2 has failed to have a voltage drop below .2 volts for 7 seconds during a deceleration fuel cut off. Because of this failure, the engine control module would determine that the sensor response is too slow.
It is with the help of powertrain control module that the fuel is cut to the engine upon deceleration, and the O2 sensors should respond accordingly by a voltage output lower than 0.2V, showing exhaust stream as high in O2 content. This code would be triggered if the O2 sensor for Bank 1 sensor 2 does not respond to the fuel cut off within 7 seconds.
What Does Code P0139 Mean?
Diagnostic trouble code (DTC) P0139 stands for “O2 Sensor Circuit Slow Response (Bank 1, sensor 2).” It indicates that the powertrain control module (PCM) has detected a potential issue with the signal from the rear oxygen sensor on bank 1.
If the O2 sensor output is sluggish, the ECM/PCM knows it can’t be trusted to provide real time accurate information.
The rear sensor is located behind the catalytic converter, and bank 1 refers to the side of the engine that houses the #1 cylinder.
In GM automotive terminology, “cross-counts” refers to the oxygen sensor’s voltage activity as the device alternates between a state of high voltage representing a rich mixture and the opposite low voltage state that indicates a lean mixture.
A sensor must produce a minimum amount of cross-counting activity to prove that it works properly.
The downstream or rear oxygen sensor doesn’t fluctuate back and forth like the front sensor. But the rear sensor’s signal should vary somewhat over time. If the rear sensor’s signal is slow to change or does not change at all, the PCM may log code P0139.
In other words, the ECM/PCM knows how rapidly the downstream O2 sensor should be switching based on the faster switching upstream sensor. It’s a rationality check algorithm.
Potential Causes of a P0139 Code
- Worn-out Oxygen Sensor: The oxygen sensor in Bank 1 Sensor 2 may be worn out or aged, leading to a sluggish response time.
- Contamination: Accumulation of dirt, oil, or soot on the sensor’s surface can impede its response and accuracy.
- Exhaust System Leaks: A leak in the exhaust system near the oxygen sensor can cause false readings and trigger the code.
- Faulty ECM: In rare cases, a malfunctioning ECM may not process the sensor’s signals correctly, leading to the code.
P0139 Code Symptoms
When the P0139 code triggered, the following symptoms may be observed:
- Check Engine Light: The most common symptom is the illumination of the Check Engine Light on the vehicle’s dashboard.
- Increased Emissions: A sluggish oxygen sensor can lead to increased emissions and environmental pollution.
- Reduced Fuel Efficiency: The engine may experience reduced fuel efficiency due to improper air-fuel ratio adjustments.
- Poor Performance: The vehicle may exhibit hesitation, rough idling, or lack of power during acceleration.
How to Fix P0139
- Scan the codes and documents freeze frame data
- The oxygen sensor data should be monitored to see if the voltage is dropping to below .2 volts during deceleration
- Carefully inspect the engine fuel pressure for a leaking fuel injector
- Check that the oxygen sensor is not contaminated with coolant or oil on the outside
- The integrity of the exhaust system should be verified for any problems with the catalyst
- The manufacturer’s specific pinpoint tests should be followed for further diagnosis
you may also try the diagnostic method below:
- Code P0139 that points to a problem with the downstream O2 sensor, which is lazier than the upstream sensor on a system where the catalyst is storing oxygen correctly.
- If you’re dealing with any kind of concern you believe might be related to the upstream O2 sensor performance and you believe the sensor might need replacing, you can watch the sensor on the scan tool data screen with the throttle at about 1200 rpm and you should see about three switches from rich to lean per second, with the voltage range being at least from 0.2 to 0.8 volts or slightly greater. The downstream sensor will switch as well, only not as much and not as quickly.
- The O2 sensor will switch more slowly at idle much of the time, so you’ll need to monitor the sensor with the engine speed elevated and even record it while driving with the engine at highway cruise.
- Again, either record the pattern while driving using the scan tool record utility or have an assistant driving. DO NOT watch the scan tool screen while driving the vehicle.
- The downstream sensor also needs to switch quickly whenever it needs to, and the ECM watches both sensors very carefully to determine if either sensor is responding to fuel trim adjustments at a less-than-optimum rate.
How Much Does It Cost to Fix a P0139 Code?
To fix a P0139 code, which typically indicates a faulty oxygen sensor, you can expect to pay between $200 and $500 for the replacement, including labor costs, depending on your car’s make and model; the majority of the cost will likely be attributed to the new oxygen sensor part itself.