How is fault code P0139 fixed?

A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training. 1. Pull the codes and watch the rear sensor live data. A good sensor swings reasonably quickly when you change throttle. If it crawls between voltages and lags well behind the front sensor, that's your slow response confirmed 2. Get under the car and check the sensor plug and wiring properly. Look for green corrosion on the pins, chafed insulation near the heat shield, and any sign water's been sitting in the connector 3. Listen and feel along the exhaust ahead of the rear sensor for leaks with the engine running. A small leak before sensor 2 mimics this code exactly 4. Measure the heater circuit resistance cold and compare to the manual figure, typically a few ohms. An open or out-of-spec heater means slow warm-up and a slow signal 5. Check resistance through the signal and ground wires back to the ECU connector to rule out a high-resistance fault before you condemn the sensor 6. If wiring, exhaust and heater all check out, the sensor itself is almost certainly worn out and due for replac

O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)

Jump to the fix

Severity

Medium

Can I keep driving?

Usually drivable, but get it diagnosed without delay.

First step

A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training. 1. Pull the codes and watch the rear sensor live data. A good sensor swings reasonably quickly when you change throttle. If it crawls between voltages and lags well behind the front sensor, that's your slow response confirmed 2. Get under the car and check the sensor plug and wiring properly.

PPowertrain
0Generic code
1Fuel / Air
39Sequence number

What does it mean?

OBD II fault code P0139 is defined as “Oxygen Sensor Slow Reponse (Bank I Sensor 2)”, and is set when the PCM (Powertrain Control Module) detects that Heated Oxygen Sensor #2 is not operating within predefined parameters.Note that “Slow Response” in the definition refers to the time taken by the sensor to react to changes in the composition of the exhaust gas.

In this definition, “Bank 1” refers to the bank of cylinders that contains cylinder #1, while “Sensor #2” refers to the oxygen sensor that is located downstream, or after the catalytic converter. The distinction between the functions of the oxygen sensors located before and after the catalytic converter(s) is an important one.

While sensor #1 (located before the converter) is primarily concerned with analyzing the exhaust stream to detect changes in the level of oxygen in the gas so that changes to short-term fuel trim can be made by the PCM, the #2 sensor (located after the converter), is concerned with monitoring the function of the catalytic converter.

In practical terms, a fully functional #2 oxygen sensor fulfills a diagnostic role, and once it has entered closed loop operation, which is when its heating element has reached its proper operating temperature of around 316 °C (600 °F), it should deliver a relatively steady signal voltage of between 0.5 volt, and 0.7 volt.

Major fluctuations in this signal voltage indicate a defective catalytic converter, since changes in the composition if the exhaust gas are not being absorbed, or “damped out” by the converter. Conversely, signal voltages from the #2 sensor that fluctuate from much below about 0.5 volt to much above 0.7 volt, or a signal voltage that remains static without changing at all indicate a defective sensor, or a fault in the sensor’s control and/or signal circuit.

The image below shows the typical appearance of an oil-fouled oxygen sensor. Excessive oil burning is a common cause of oxygen sensor related codes, which is more common on applications that are known for high oil consumption rates.

Symptoms

Likely causes

How to fix it

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This information follows the generic OBD-II definition and does not replace a vehicle-specific diagnosis. The severity rating is rule-based; an authorised diagnostic reading is required for a definitive answer.