P0161 OBD-II: O2 Sensor Heater Circuit

What Does P0161 Code Mean?

OBD-II Code P0161 is defined as a Oxygen Sensor Heater Circuit Malfunction (Bank 2, Sensor 2)

The rear Oxygen Sensor is located in the exhaust system behind the catalytic converter(s). It sends critical feedback data to the Power Train Control Module or (PCM) that tracks the operational efficiency of the Catalytic Converter. It collects this data by measuring the amount of oxygen in the exhaust gases leaving the Catalytic Converter. Oxygen Sensors have their own electrical heating element that maintains the optimal operating temperature that ranges from 300 to over 600 degrees F.

Code P0161 sets when the Powertrain Computer or PCM has determined that the Oxygen Sensor Heating Circuit is malfunctioning. This means that the Heating Circuit is using too little or too much electrical power.

P0161 Symptoms

• Check Engine Light will illuminate
• Vehicle may idle or run rough
• Decrease in fuel Economy because PCM is in a "limp home" mode
• Engine may die
• In some unusual cases, there are no adverse conditions noticed by the driver

Common Problems that Trigger the P0161 Code

• Defective Oxygen Sensor
• Defective Oxygen Sensor Heater circuit
• Defective sensor wiring and/or circuit problem
• PCM software needs to be updated
• Defective PCM

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
• CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
• NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

**P0161 Diagnostic Theory for Shops and Technicians:

Oxygen Sensor**

When the code P0161 is set, record the freeze frame data in fine detail. Next, duplicate the code setting conditions on a test drive, paying particular attention to load, MPH, and RPM. The best tool to use on this test drive is a data streaming scan tool that has dedicated factory quality live data. Be sure to verify the code conditions before you advance to the next set of tests.

_ If You Cannot Verify the Code Setting Malfunction _
If you cannot verify the code setting malfunction, then do a careful visual inspection of the sensor and the connections. Verify that there are 12-volt heater signal(s) and good ground(s) to the sensor and that they energize at the required times, as per the manufacturer diagnostic documentation. Verify that the signal from the Oxygen Sensor to the PCM is being "seen" by back probing the Oxygen Sensor connector and, if needed, back probing the signal wire at the PCM. Inspect the sensor harness to ensure that it isn't chafed and/or grounding anywhere and be sure to perform a wiggle test. You will want to use a high impedance Digital Volt Ohm Meter (DVOM) for all of these electrical tests. If you still cannot find a problem, then try these steps next:

• If you can receive authorization from the customer to keep the vehicle overnight, clear the code and test drive the vehicle by driving it home and then back to work in the morning, making sure that you are duplicating the code setting driving conditions on both trips. If the code still does not come back, you can give the customer the option of replacing the Oxygen Sensor as a diagnostic step since the sensor is the most likely problem and the code will presumably set again. If the customer declines, then return the vehicle with a clear description of the inspections and your findings plainly attached to the final copy of the repair order. Keep another copy for your own records in case you have to re-visit this inspection for any reason.

• If this is an inspection for an emissions failure, most government programs suggest that you replace the sensor as a preventative measure so the vehicle won't remain in a highly polluting operational condition. After the Oxygen Sensor is replaced, the monitors will have to be re-set and this, too, will test most phases of the Oxygen Sensor system to ensure that the problem was solved. Be sure to verify that the Mode 6 test IDs and component IDs that pertain to fuel control are well within the parameter limits. If there is a problem with re-setting the monitors, continue the inspection until you find the root cause of the problem.

If You Can Verify the Code Setting Malfunction
If you can verify the code setting malfunction, then do a careful visual inspection of the sensor, the connections, and the exhaust system. Verify that there are 12-volt heater signal(s) and good ground(s) to the sensor and that they follow the required on times, as per the manufacturer diagnostic documentation. Verify that the signal from the Oxygen Sensor to the PCM is being "seen" by back probing the Oxygen Sensor connector and, if needed, back probing the signal wire at the PCM. Inspect the sensor harness to ensure that it isn't chafed and/or grounding anywhere and be sure to perform a wiggle test. You will want to use a high impedance Digital Volt Ohm Meter (DVOM) for all of these electrical tests.

• The most comprehensive way to test and condemn an Oxygen Sensor Heater Circuit is to use a Dual Trace Labscope with the time division graticule set at 100-millisecond intervals and the voltage scale set at +/- 2 volts. Run the warmed-up vehicle with the signal wire back probed and watch to see if the signal sticks above 900 millivolts and for how long. Do this while the engine is idling and at 2000 RPM. A properly working Oxygen Sensor should switch from lean (less than 300 millivolts) to rich (above 750 millivolts) in less than 100 milliseconds and should do it consistently. The voltage should only stick in a very high mode during wide open throttle operation, when the system is in open loop.

• Next, perform a range test and time test, still using the Labscope. Run the engine at 2000 RPM and quickly close the throttle and then snap it back open. The Oxygen Sensor Signal needs to go from around 100 millivolts (when the throttle closes) to above 900 millivolts (when the throttle opens) in less than 100 milliseconds. A new sensor will do this test within these ranges in less than 30–40 milliseconds. A good sensor should never stick in a really high mode.

• If the sensor fails either of the above Labscope inspections, most emissions programs will allow you to condemn the sensor because the slow switching time leads to high NOx levels and above-normal CO levels and HCs. This is because the Cerium bed of the OBD II Catalytic Converter is not being supplied with the proper amount of Oxygen each time the signal "lags" between the peaks and valleys of its sine wave.

Note: If the Oxygen Sensor signal ever goes to a negative voltage or above 1 volt, this alone is enough to condemn the sensor. These out-of-range readings are often caused by the Heater Circuit bleeding voltage or ground into the Oxygen Sensor signal circuit. They can also be caused by contamination or physical damage to the sensor.

• If the above tests and inspections don't produce verifiable results, then physically remove the Oxygen Sensor. If the Sensor Probe has a white and chalky appearance, the sensor has been lagging between switching phases and needs to be replaced. It should have the light tan color of a healthy spark plug.