The days of the zirconia upstream oxygen sensor are behind us. Vehicle manufacturers have moved to what is commonly known as the air/fuel ratio sensor, or AFS. With the change in technology has come a steep increase in the price of the replacement part, which means you need know how to diagnose sensor and air/fuel related faults. This month, we will focus on Asian AFS operation, diagnosis and replacement with a focus on what you need to know to fix it right the first time.
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When a spark ignition engine is in open loop operation, it is using a feed-forward strategy for operation. The Engine Control Module (ECM) is looking at inputs such as engine load and RPM, as well as adjustments for air and coolant temperature and is commanding an injector pulse width to match fuel to the incoming air at a specified ratio. Typically the AF ratio being commanded is 14.7 parts of air to 1 part of fuel by weight. This ratio is commonly known as stoichiometric ratio. While it has been commonly taught that the ECM is primarily commanding stoich, it is not a fixed rule.
In closed loop operation, the AFS is used to provide feedback to the ECM and reports the actual air fuel ratio to the ECM. While the AFS is the primary sensor for feedback, the ECM uses the downstream oxygen sensor to make secondary adjustments for the catalyst.
For years we have learned about the zirconia oxygen sensor. We were taught that 450 millivolts was stoich, right? Well, it is, but so is anything in the range of 200-800 millivolts. Yes, you read that correctly. The zirconia oxygen sensor’s range is typically in the area of 100-900 millivolts and as such the ECM/PCM doesn’t truly know how rich or how lean the air fuel mixture is. Until the sensor reaches the 200 millivolt range on the lean end and 800 millivolts on the rich end the sensor is assuming stoich. These of course are not hard numbers as the range will vary slightly with sensor temperature and sensor manufacturer. In order to maintain stoich on a system with an upstream zirconia type O2 sensor, the ECM had to toggle the fuel command rich and lean in order to keep the AF ratio within the 200-800mV range. This toggling of AF ratio was an inefficient way of maintaining stoich. The zirconia style oxygen sensor can only accurately see a very narrow range of air/fuel ratios. In fact, it really only sees stoich. Again, it cannot accurately report how rich or how lean the mixture is.
The AF Sensor is considered a wide-range sensor. What does that mean exactly? It means that it can accurately see a wide range of air/fuel ratios. This has become increasingly important to manufacturers as they try to improve fuel economy and tailpipe emissions. The ability to see the exact AF made the switch to the AFS a no brainer. With the accuracy of such a sensor comes the ability to command a variety of fuel ratios for specific engine load and operating conditions. Keep in mind that AF sensors may look similar to their O2 sensor counterparts but they function much differently.
Sensor voltage? Amperage?
The most important difference between the AFS and the O2 are how they operate internally. My advice would be to focus on how to diagnose these sensors and don’t get caught up in their overall operation. While they all achieve a wide-range sensor result they all are designed by different engineers and different manufacturers. Attempting to memorize sensor voltages is a bit overwhelming.
While the zirconia O2 was a voltage generator, the AFS actually outputs a current flow (amperage). When the engine is running at stoich there is no current flow. When the engine is running lean the current flow increases. When the engine is running rich the current flow polarity changes and you get a negative current flow value. The ECM then takes these values and converts them into a voltage. The voltage range will vary from manufacturer to manufacturer with no standard voltage for stoich. For example, Toyota uses 3.3 volts as stoich. Anything below 3.3 is rich and anything above 3.3 is lean. Each manufacturer’s voltage value can be treated the same.
The question I am always asked is which value should be used in diagnosis. The answer is whichever one you are comfortable with. In the Techstream screenshot provided I graphed both AFS voltage and AFS current. You will see the graphs are identical. Why? Because the ECM converted the current into a readable voltage value. Keep in mind the voltage output of an AFS is opposite to that of the zirconia oxygen sensor. Values higher than stoich voltage are lean and values lower than the stoich voltage are rich.