What does it mean?
OBD II fault code P0159 is defined as “Oxygen Sensor Slow Response (Bank 2 Sensor 2)”, and is set when the PCM (Powertrain Control Module) detects a voltage or signal from or to the #2-oxygen sensor that does not fall within predefined specifications. 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 2” refers to the bank of cylinders that does not contain 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.