Before we get into theoretical details let’s make the following clear:
P0135 or P0141 heater circuit DTCs are almost always defective sensors that can be checked by using Ohms on your meter. “OL” indicates that an open heater circuit exists in the sensor and you should replace it.
A sensor obviously dead in the water not giving any feedback is most likely not a wiring problem. The easiest way to confirm this is to backprobe the sensor itself and look to see if it is showing voltage on your meter or labscope. Oxygen sensors generate their own voltage and if they show nothing they are obviously bad. Try taking one sensor out of a car and put it to the shop torch. You’ll see it makes its own voltage. (An air-fuel sensor also generates its own voltage, but it cannot be tested this way.)
Stick with the OE brand sensor. I have seen aftermarket sensors functionally be perfect with a good signal and working heater circuits, but they still set DTCs. Ignore the parts guy and just get the right sensor. Most Asian vehicles take Denso (sometimes NTK). Older American vehicles usually have Bosch but they have mostly moved over to Denso as well. European vehicles mostly use Bosch. Walker does not make their own sensors, but at a guesstimated 80% clip they rebox the OE sensor. If you are not sure what sensor the vehicle came with (and you cannot read it on the outside of the sensor) either buy it from the dealer first or remove it, bring it to the parts tore or dealer and match it up. Often you can buy the OE brand through the aftermarket, as long as you stick with the brand you took out of the vehicle.
Getting to know the oxygen sensor
The oxygen sensor measures the amount of oxygen in the exhaust that is used in the combustion process.
For pre-catalytic converter oxygen sensors used for fuel control:
Less oxygen than optimal in the exhaust results in a signal voltage over 450 mV. This reflects a RICH CONDITION. More oxygen in the exhaust than optimal results in a signal voltage under 450 mV. This reflects a LEAN CONDITION.
Good oxygen sensors have even waves in the 150 mV to 850 mV range while ascending or descending within a 100 mS or less when the system is in closed loop.
For post-catalytic converter oxygen sensors used for fuel control:
Post-cat oxygen sensors, when good, feature a steady voltage usually between 500 to 700 mV. If it zigzags, the catalytic converter is highly suspect.
On some vehicles the rear sensor does have some effect on fuel control. For our purposes, it’s just good to know that when testing the sensor, the voltage should go up when the fuel mixture is rich and should go down when it is lean. Sadly there is no way of generically knowing what is an optimal post-cat oxygen sensor voltage. It differs by the manufacturer.
Both front and rear oxygen sensors can be tested in the same way:
To make sure the sensor is reacting as it should to rich and lean conditions, simply cause a vacuum leak to make the system lean and use some propane to make the system run rich. You can do all of this by simply pulling out the brake booster hose. After you do this be sure to pump the brakes a couple times after you put everything back together. The sensor should react to rich and lean conditions instantly. If not, you might have a “lazy” sensor that needs to be replaced.
Mode 5 and Mode 6 tests
Even though Mode 5 is pretty much a thing of the past, both Mode 5 and 6 work the same. All they do is tell us if the PCM is happy with the feedback the oxygen sensors are giving it.
Mode 5 is not available on all vehicles besides some pre-CAN ones, but when it is you should view the data. The figures show how both Mode 5 and 6 provide voltage readings and switching results. The results can be helpful in making a decision concerning a P0420 DTC. If the front oxygen sensor voltage is not low or high enough, and is not switching in the correct time, you may not want to condemn that converter. When Mode 5 is not available Mode 6 should be used to view oxygen sensor test data.
Differences between the oxygen and air-fuel sensor
While both are used to measure catalytic efficiency and determine if a vehicle is running rich or lean, the way they work is fundamentally different. Air-fuel sensors reflect a lean condition when their voltage INCREASES and a rich condition when their voltage DECREASES.
Air-fuel sensors are only used for fuel control, so they are always a pre-catalytic converter sensor, not a post-catalytic converter sensor. The post-cat sensor is always a standard oxygen sensor. While a pre-cat oxygen sensor switches voltage from rich to lean, the air-fuel sensor stays at a steady voltage.
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