P0139 code definition
O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)
What the P0139 code means
P0139 is the OBD-II generic code indicating that the O2 sensor for bank 1 sensor 2 failed to have a voltage drop below .2 volts for 7 seconds during a deceleration fuel cut off. This failure shows that the engine control module (ECM) has determined the sensor response is too slow.
What causes the P0139 code?
The ECM cuts all fuel to the engine during deceleration of the engine and the O2 sensors all should respond with a voltage output below .2 volts showing the exhaust stream is high in oxygen content. The code is set if the O2 sensor for bank 1 sensor 2 does not respond to the fuel cutoff for 7 seconds or more.
This can be caused by excessive fuel still in the exhaust stream from possible leaking in the fuel injection system.
What are the symptoms of the P0139 code?
The engine may stall or run rough on deceleration if excessive fuel is getting in the engine.
The Check Engine Light will come on.
The engine may hesitate upon acceleration after a deceleration phase.
How does a mechanic diagnose the P0139 code?
Scans codes and documents freeze frame data
Monitors the O2 sensor data to see if the voltage is dropping to below .2 volts during deceleration
Check the engine fuel pressure for a leaking fuel injector
Verifies the O2 sensor is not contaminated with coolant or oil on the outside
Verifies the integrity of the exhaust system for any problems with the catalyst
Follows the manufacturer's specific pinpoint tests for further diagnosis
Common mistakes when diagnosing the P0139 code?
Follow these simple guidelines to prevent misdiagnosis:
The slow response of the sensor can be caused by a leaking fuel injector on bank 1 of the engine if both sensors 1 and 2 on that side are having the same problem.
A stuck-open throttle can stop the fuel cut off phase and should be fixed before this code.
Check the exhaust catalyst for any damages that could cause the sensor to not work properly.
How serious is the P0139 code?
This code, if the sensor is not bad, indicates the engine is still pumping fuel on deceleration when no fuel is needed. This can increase the fuel usage and may cause the engine to stall when coming to a stop if excessive fuel is leaking into the cylinders.
The ECM cannot control the fuel shut off if the fuel injectors are leaking and also will lead to excessive fuel consumption.
What repairs can fix the P0139 code?
The O2 sensor for bank 1 sensor 2 should be replaced only after all other checks of the fuel and exhaust systems test okay.
Testing the fuel system and replacing a leaking injector
Replacing the catalyst in front of the sensor
Cleaning the injectors to see if the leaks stop before replacing them
Additional comments for consideration regarding the P0139 code
The slow response of an O2 sensor may be due to a sensor that is just getting old and over time may get contaminated with time with carbon and other contaminants that deteriorate the sensor over time.
Need help with a P0139 code?
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