P0139 - HO2S Bank 1 Sensor 2 O1 Sensor Circuit Slow Response (B2SXNUMX)

P0139 – trouble code description

The heated oxygen sensor 2 (ho2s), downstream of the three-way catalytic converter (manifold), monitors the oxygen level in the exhaust gases of each cylinder bank. For optimal catalyst performance, the air to fuel ratio (air-fuel ratio) must be maintained close to the ideal stoichiometric ratio. The output voltage of the ho2s sensor suddenly changes near the stoichiometric ratio.

The engine control module (ECM) adjusts the timing of fuel injection so that the air-fuel ratio is nearly stoichiometric. In response to the presence of oxygen in the exhaust gases, the ho2s sensor generates a voltage of 0,1 to 0,9 V. if the oxygen content of the exhaust gas increases, the air-fuel ratio becomes lean.

The ECM module interprets a lean mixture when the ho2s sensor voltage is less than 0,45V. if the oxygen content of the exhaust gases decreases, the air-fuel ratio becomes richer. The ECM module interprets the rich signal when the ho2s sensor voltage exceeds 0,45V.

What does DTC P0139 mean?

Trouble code P0139 is associated with the driver's side rear oxygen sensor and indicates that the engine's air-fuel ratio is not being adjusted correctly by the oxygen sensor or ECM signal. This may occur after the engine has warmed up or when the engine is not operating normally. “Bank 1” refers to the bank of cylinders that contains cylinder #1.

Code P0139 is a common OBD-II standard and indicates that the bank 1 oxygen sensor, sensor 1, did not exhibit a voltage drop of less than 0,2 volts for 7 seconds during the fuel latch period. This message indicates a slow sensor response as detected by the Engine Control Module (ECM).

Possible reasons

For code P0139, the ECM reduces fuel supply to the engine during engine deceleration and all O2 sensors should respond with an output voltage of less than 2 V, indicating high oxygen content in the exhaust gases. An error code is set if the bank 2 O1 sensor, sensor 1, does not respond to a fuel cut for 7 seconds or more.

This may be caused

• excess fuel in the exhaust gas flow due to possible leaks in the fuel injection system,
• malfunction of the rear heated oxygen sensor, block 1,
• Rear heated oxygen sensor bank 1 wiring harness (open or shorted),
• problems with the electrical connection of the rear heated oxygen circuit 1 battery,
• insufficient fuel pressure,
• faulty fuel injectors,
• air leaks in the intake,
• defects in the oxygen sensor unit with reverse heating,
• Rear heated oxygen sensor bank 1 wiring harness (open or shorted),
• malfunction of circuit 1 of the rear heated oxygen sensor,
• insufficient fuel pressure,
• faulty fuel injectors and possible malfunction in intake air leaks,
• as well as exhaust gas leaks.

What are the symptoms of the P0139 code?

• The engine may stall or run rough due to excess fuel.
• The engine may exhibit hesitation during acceleration after deceleration.
• The check engine light (or engine maintenance light) comes on.
• High fuel consumption.
• Excessive amount of smoke in the exhaust system.

How to diagnose code P0139?

1. Scan codes and data records, capturing information from the frame.
2. Monitor the O2 sensor reading to determine if the voltage drops below 0,2 V during deceleration.
3. Check engine fuel pressure for leaks in the fuel injector system.
4. Make sure the O2 sensor is not contaminated by external substances such as coolant or oil.
5. Inspect the exhaust system for damage or problems, especially in the catalytic converter area.
6. Perform the tests provided by the manufacturer for additional diagnostics.

Diagnostic errors

To avoid misdiagnosis, follow these simple guidelines:

If both sensors (1 and 2) on the same side of the engine are slow to respond, pay attention to a possible fuel injector leak in the first bank of the engine.

Before this code occurs, troubleshoot any possible problems with a stuck throttle valve that may interfere with the fuel shutoff process.

Check the condition of the catalytic converter for damage that could cause the sensor to malfunction.

How serious is trouble code P0139?

This code indicates that even if the sensor is good, the engine still continues to deliver fuel during deceleration, even when it is not required. This can lead to increased fuel consumption and even the engine stalling when stopped if too much fuel is entering the cylinders.

The ECM (engine control module) is unable to control fuel shutoff if the fuel injectors are not sealed, which may cause excessive fuel consumption.

What repairs will fix the P0139 code?

Replacement of the O2 sensor for bank 1 sensor 1 should only be performed after all other fuel and exhaust system checks have been completed.

1. First, check the condition of the fuel system and replace a leaking fuel injector if found.
2. Replace the catalyst in front of the sensor if it is faulty.
3. Before replacing the O2 sensor, clean the injectors and make sure any leaks have been repaired.

A slow O2 sensor response may indeed be due to aging and contamination. Since the O2 sensor measures the oxygen content of exhaust gases, any deposits or contaminants on its surface may interfere with the correct measurement. In such cases, cleaning or replacing the sensor can help restore its functionality and improve its response to changes in exhaust gases.

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