P0159 trouble code definition

Oxygen Sensor (O2) Circuit Slow Response (Bank 2 Sensor 2)

What the P0159 code means

Code P0159 is a powertrain code that indicates a specific sensor in the exhaust system (bank 2 sensor 2) is not performing as it should. When an oxygen sensor modulates slowly, it is an indication it is bad. This sensor monitors the catalyst efficiency for emissions purposes.

What causes the P0159 code?

This code is set if the oxygen sensor modulates slowly. It should fluctuate between 800mv and 250mv 16 times in a twenty second period. If the sensor does not perform to this standard, it is considered to be bad. Usually due to age or contamination.

Exhaust leaks will set this code as well. Contrary to popular belief, an exhaust leak will suck in oxygen diluting the exhaust flow that will simulate the conditions the computer interprets as a bad oxygen sensor.

This is a four wire sensor with two circuits. If either of these circuits has a short or high resistance, this code could be set as these conditions will affect the oxygen sensor’s ability to perform properly.

What are the symptoms of the P0159 code?

A Check Engine Light. This sensor is mainly for emissions purpose and does not have a significant effect on the performance of the vehicle.

This is a downstream oxygen sensor, meaning it is after the catalytic converter. The computer utilizes downstream oxygen sensors to monitor catalyst efficiency and upstream oxygen sensors to calculate air fuel mixture.

How does a mechanic diagnose the P0159 code?

Technical Service Bulletins (TSB’s) should be checked for problems specific to individual year make and model vehicles.

The computer sets this code after performing some very specific tests. For this reason, a technician having scanned the vehicle and finding this code will likely confirm there aren’t any exhaust leaks before replacing the indicated sensor (bank 2 sensor 2).

If more extensive tests should be necessary, there are variety of methods to do so. A technician can tap directly into the oxygen sensors circuit and observe its function with an oscilloscope. This is usually performed while introducing propane into the intake or creating a vacuum leak and observing how the oxygen sensor reacts to the change in conditions. These tests are often done in conjunction with a test drive.

Resistance tests can be taken with the oxygen sensors connector unplugged from the vehicle's electrical harness. At times, this is done while applying heat to the sensor simulating conditions the sensor would experience when installed in the exhaust system.

Common mistakes when diagnosing the P0159 code

Failing to spot other problems, such as exhaust leaks, vacuum leaks, or misfires are not uncommon. Sometimes other problems can be subtle and easy to miss.

Downstream O2 sensors (O2 sensors after the catalytic converter) exist to make sure the vehicle meets the EPA’s tailpipe emissions standards. Not only does this O2 sensor test the catalyst efficiency, but tests are performed to test the effectiveness of the sensor itself.

The stringent nature of these tests require all other systems to be operating correctly or the results may not be accurate. For this reason, most other codes and symptoms should be addressed first.

How serious is the P0159 code?

This code has a low impact on the daily driving of the vehicle. This is not a system that will make a tow truck necessary.

The seriousness of global warming is what has prompted the EPA and auto manufactures to implement systems such as this.

What repairs can fix the P0159 code?

  • The most common repair is the replacement of the indicated oxygen sensor (bank 2 sensor 2).

  • Exhaust leaks must be repaired before any oxygen sensor is replaced.

  • It is not uncommon to find damaged wiring in the oxygen sensor circuit that would need to be repaired. These circuits are shielded and require extra attention when fixing them.

Additional comments for consideration regarding the P0159 code.

  • Bank 1 is the set of cylinders with number one cylinder in it.

  • Bank 2 is the set of cylinders that does not have number one cylinder.

  • Sensor 1 is the sensor before the catalytic converter which the computer uses to make fuel ratio calculations.

  • Sensor 2 is the sensor that comes after the catalytic converter and is mainly used for onboard emissions testing.

The following conditions must be met for your car to test the health of the bank 2 sensor 2 O2 sensor. This fault detection method may vary between manufacturers and will occur when the following conditions exist:

  • When the car is moving between 20 and 55 miles per hour.
  • The throttle must be open for a minimum of 120 seconds.
  • Operating temperature above 70? (158?).
  • Catalytic converter temperature above 600? (1112?).
  • The evaporative emissions system is purging.
  • The code is set if the oxygen sensor voltage transitions less than 16 times from rich to lean in a 20 second interval.
  • This test utilizes two trip fault detection.

Need help with a P0159 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

This article originally appeared on YourMechanic.com as P0159 OBD-II Trouble Code: Oxygen Sensor (Bank 2 Sensor 2) and was authored by Robert Kulp.


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