P0139 Code: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)

Oxygen sensors switch between high and low voltage signals that correspond to the oxygen levels in the exhaust system. However, the signal from the front sensor (sensor 1) changes far more rapidly than that of the rear (sensor 2), and both change over time. When the rear oxygen sensor mounted on engine bank 1 doesn’t transition between voltage states fast enough, the PCM triggers the P0139 code. This article explains the symptoms, causes, and repair costs of the P0139 code.

P0139 Code Definition

Diagnostic trouble code (DTC) P0139 stands for “O2 Sensor Circuit Slow Response (Bank 1, sensor 2).”

What Does the P0139 Code Mean?

The P0139 code indicates that the powertrain control module (PCM) has detected a potential issue with the signal from the rear oxygen sensor (sensor 2) on bank 1.

Sensor 2 is located behind the catalytic converter, and bank 1 refers to the side of the engine that houses the #1 cylinder.

P0139 code  

In GM automotive terminology, “cross-counts” refers to the oxygen sensor’s voltage activity as the device alternates between a state of a 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.

Symptoms of the P0139 Code

Following are the most common symptoms of the P0139 code:

  • The engine may stall or run rough on deceleration if excessive fuel gets in the engine.
  • The engine may hesitate upon acceleration after a deceleration phase.
  • The Check Engine Light illumination.
  • A reduction in engine performance.
  • Failed emissions test 

Read More: Symptoms and Causes of P0451

Causes of the P0139 Code

Following are the major causes of the P0139 code:

  • Bad bank 1 sensor 2 oxygen sensor
  • Problems with the rear oxygen sensor wirings or connections
  • Exhaust system leakage
  • An issue with the PCM, such as software in need of an update 
  • A rich or lean running condition  

Read More: Symptoms and causes of bad PCM

How to 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
  • Properly inspect the O2 sensor and clean it if needed
  • Verifies the integrity of the exhaust system for any problems with the catalyst
  • Properly inspect the wirings and repair them if they are damaged
  • Follows the manufacturer’s specific pinpoint tests for further diagnosis

Common P0139 Code Diagnostic Mistakes

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.
  • Inspect the exhaust catalyst for any damages that could cause the sensor not to work properly.
  • A stuck-open throttle can stop the fuel cut-off phase and should be fixed before this code.

What repairs can fix the P0139 code?

  • The O2 sensor for bank 1 sensor 2 must be replaced only after all other checks of the fuel and exhaust systems test okay.
  • Inspect and replace the catalyst in front of the sensor.
  • Test the fuel system.
  • Inspect the fuel injector. If it is leaky, then repair or replace it.
  • Clean the injectors to see if the leaks stop before replacing them.

For a better understanding of how to fix the P0139 code, watch the below-given video:

P0139 Code Repair Cost

The repair or fixing cost of the P0139 code varies according to the nature of the repair, vehicle model, and labor cost. The average replacement cost of an O2 sensor is from $340 to $394In this cost, the parts costs are from $270 to $320, while the labor costs are from $70 to $74

Parts Cost$270 to $320
Labor Cost$70 to $74
O2 sensor Replacement Cost$340 to $394

FAQ Section

How serious is the P0139 code?

  • If the sensor is not bad, this code indicates the engine is still pumping fuel on deceleration when no fuel is needed. This can increase 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, which also will lead to excessive fuel consumption.

What is an O2 sensor bank 1 sensor 2?

Bank 1 Sensor 2 is the oxygen (O2) sensor that’s in the middle of the catalytic converter or right behind the front catalytic converter (the converter that’s closest to the engine).

What causes low voltage in the oxygen sensor?

A low voltage signal from the oxygen sensor indicates low oxygen content in the exhaust—a sign that the engine runs rich. The PCM responds by reducing the fuel going to the engine.

Where is the oxygen sensor bank 2 located?

A vehicle’s (bank 2, sensor 2) O2 sensor will be located behind or downstream of the catalytic converter on the bank, which corresponds to the engine’s second cylinder.

Is bank 2 driver or passenger side?

Bank 2’s location varies in every automobile. It depends on its make, model, and driver orientation (left-hand or right-hand drive). It would be presumptive to simply say it is found on the passenger side.

Which side is bank 2 sensor 2?

All oxygen sensors are located on the exhaust system; Bank 2 for your engine will be the bank of cylinders closest to the front of the car. Sensor 2 will be the second oxygen sensor downstream on that side of the exhaust pipes after the catalytic converter.

Is bank One sensor 2 upstream or downstream?

Sensor 1 is the upstream oxygen sensor. It is the sensor that measures the oxygen content in the exhaust, providing input to the computer, which determines how to adjust the air/fuel ratio. Sensor 1 is the sensor closest to the engine. Sensor 2 is the downstream oxygen sensor.

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