BulletinTB-80041
from Gary Stamberger / MagnaFlow Training Director

They appear in our trade magazines, diagnostic aids and online chats under many names: Oxygen Sensor, LAF, UEGO, A/F Sensor, Lambda Sensor, Planar, Wide Band or Wide Ratio Sensor. The list seems to go on ad infinitum. No matter what we call them, their function remains the same, to aid the computer in getting the most out of every drop of fuel.

Beginning in the late 1990’s we began to see a shift in how closed loop fuel control systems were operating. The change came in the form of a wide range sensor (aka air fuel sensor). Although the introduction was slow at first, mostly Honda and Toyota, the use of these variations on the standard zirconia O2 have become widely accepted.

The job description of “supplying feedback information about the oxygen content of the exhaust to the PCM” hasn’t changed but how we diagnose these new sensors is much different. The tool we use will very much dictate the results of our testing and many times might cause the diagnosis to be incorrect.

The advantage of a Wide Band sensor is two fold. First it has the ability to react quickly to changes in Air Fuel Ratio and therefore is better capable of keeping the AF at Stoichiometric or 14.7-1. This can also be displayed as Lambda equal to 1.

Secondly, it accurately determines exactly what your AF ratio is over a very wide range. In some cases as Rich as 7.0:1 or as lean as 20:1. With a Zirconia oxygen sensor the computer knows when it is rich or lean but not HOW MUCH!

Air Fuel sensors are not voltage generators in the same sense as an Oxygen sensor. It more closely resembles a Hot Wire MAF sensor in it’s relationship to the PCM and how we diagnose it. Testing issues revolve around checking these sensors with a scan tool in OBD II Generic mode and particularly using a scanner that may not be updated. OBD II requires that the O2 PID be displayed as a voltage ranging between 0v to 1v. An AF sensor, typically operates between 2.5v and 4.5v (this range will vary by manufacturer). A scan tool in OBD II generic mode may modify the actual voltage so as to be able to display it in a 0-1v parameter. This causes the voltage to be displayed as .65v ranging between .55v and .75v full rich to full lean. At first glance this would appear to be a dead oxygen sensor. However replacement of the sensor would leave us with the same problem.

The key then to testing Wide Band sensors is using a factory scan tool or a generic tool with factory software. Many of your newer scan tools do a good job in Enhanced Mode. When testing one of these sensors with a scanner, simply put the data in graphing mode and look at the total voltage swing. If it is between 1.0v and 5.0v then you are getting accurate data.

Another important characteristic of these wide band sensors is that they operate at double the temperature of a zirconia O2, 1200°F vs 600°F. Sometimes this information can be useful in determining whether the vehicle you are working on has an oxygen or wide band sensor. On a vehicle with a WB sensor, a quick check of the wiring diagram will reveal a relay in the heater circuit. This is necessary because of the increased current necessary to maintain the higher temperature.

These sensors are critical to Catalytic Converter operation and proper diagnosis is the key to getting the vehicle fixed right the first time. It is always worth the time to properly identify the system and all its components before beginning any diagnostic routine.