August 2002 Articles & Features
By Mark Warren | August 2002
Wide-range air/fuel ratio sensors have been around for years, but their operation may not be fully understood. Mark explains how their data is reported to your scan tool.
In 1997 I was doing a live OBD II demonstration at a trade show, for which a friend supplied a brand-new Volvo. All was fine until I started graphing the O2 sensors. They were dead flat. The vehicle had been running for some time, so I declared, "There is obviously some issue with these O2 sensors. Either they're not being reported correctly or they're dead."
A Volvo tech in the audience proceeded to point out that I was incorrect. He said that this Volvo used wide-range O2 sensors, and they do not sweep between 0 and 1 volt. Whoops! Well, we can't always be right. Good thing I was only doing a demonstration and not actually working on the vehicle for a paying customer!
Wide-range single- and dual-cell O2 sensors, and lean-air/fuel O2 sensors, have been in use since 1996. If you haven't seen one yet, you will soon. Better get up to speed now to save yourself some time later.
First, you can't just slap on a scope and get a voltage signal like with conventional O2 sensors. These sensors use a low-current signal (milliamps) supplied by the PCM to determine the air/fuel ratio. You'll need a scan tool to accurately diagnose these sensors. No sweat; O2 sensors are required OBD II reported parameters, so you can use any generic scan tool to monitor these important inputs, right? Sorry, but it's not that simple.
The original OBD II regulations and standards required that oxygen sensors report O2 readings between 0 and 1 volt (no one knew then that other types of sensors might be developed). The '97 Toyota Camry used for this column uses a wide-range O2 sensor that reports voltages from 2.4 to 4.8 volts (see Fig. 1). This represents an air/fuel ratio range from 12:1 to 20:1, with 3.3 volts being the equivalent of 14.7:1, or stoichiometry.
What was Toyota to do? The law required the scan tool to report O2 readings between 0 and 1 volt. Toyota chose to report the parameter normally for the enhanced scan tool and to divide the voltage by 5 for the generic scan tool. They did a pretty good job of explaining this in the factory manual. Also, there's a great Toyota factory technical explanation of their air/fuel sensors at this address on iATN: http://members.iatn.net/tech/toyota/sensor/sens_15.html.
The folks at the California Air Resources Board (CARB) realized this was liable to create mass confusion. Also, different manufacturers might use different schemes to comply with the regulation. So they took action to create new OBD II generic parameters for wide-range sensors, with wider voltage allowances. To avoid confusion, they required manufacturers to discontinue converting wide-range sensors to "normal" O2 voltages. This created new parameters for wide-range O2 sensors with new communication protocols. So if you're using a generic scan tool, these sensors may not report O2 readings at all until your scan tool maker updates your equipment to recognize the new protocols.
For this column, I'll discuss only the air/fuel sensor on this '97 Camry. Don't assume that all these sensors are the same. Take a look at Fig. 2 above. This is a portion of a 20-minute test drive (note the seconds labeled at the top of the graph). The meter readings in the boxes to the right of the parameter labels are from the green marker at the 520-second point in the graph. Note that the top graph of the O2 sensor reads .65 volt in the meter box. As you can see from the vehicle speed (the third graph), we maintained a steady speed from 375 to 775 seconds. So .65 volt x 5 = 3.25 volts, which is not far from 3.3 volts or a 14.7:1 air/fuel ratio normal for correct catalytic converter operation.
The maximum voltage of .92 volt was reached at the 850-second point (.92 volt x 5 = 4.6 volts, or an air/fuel ratio of about 19:1). The lowest voltage of .46 was obtained just prior to this 850-second point; this equates to .46 volt x 5 = 2.3 volts, or about a 12:1 air/fuel ratio. Spend some time examining this graph to understand the relationship among the four parameters-air/fuel sensor, rpm, vehicle speed and throttle position.
Note the poor ranging of the air/fuel sensor in the first 150 seconds of the test drive. This vehicle was setting an air/fuel sensor code. After warming up for some time, the sensor did manage to achieve its full range. This made us suspect a possible heater problem. But the heater circuit had a good waveform, which we captured using a current probe. Also, I found several TSBs relating to the sensor code. The bulletins referred to an updated sensor and PCM. Unfortunately, both updates had already been performed on this vehicle under warranty 20,000 miles earlier. To complete the analysis, we'll need more data from a properly functioning vehicle to compare to the data we have from our Camry to try and determine the problem area. Or we could just slap in a new sensor at $300 and hope for the best.
Know what the vehicle you're working on has for O2 sensors and diagnose the vehicle accordingly. Remember, these sensors still measure O2 and compute air/fuel ratio, so they're susceptible to the same errors as any O2 sensor (exhaust air leaks, misfire, etc.). We don't fully know the failure modes of these new sensors, or how they respond to lead, silicone or phosphorus contamination.
Also, be aware that the connector and appearance of this Toyota sensor is identical to the "normal" sensor, with one exception: A plastic pin is inserted in the female connector to prevent incorrect usage. My friends at the tech lines report that some well-meaning techs have pried out the pin and connected the wrong sensor, thereby "saving" the customer hundreds of dollars.
Finally, be sure to check prices and availability of the sensors and other needed parts before quoting repairs on these cars. I'll be covering more on these sensors in future issues, so stay tuned.
Lift Article Correction
The Automotive Lift Institute (ALI) has called our attention to an incorrect statement in the "Need a Lift" article in the May 2002 issue of Motor magazine. The article statement dealt with the function of the swing arm pivot restraints. According to the ANSI/ALI ALCTV-1998 standard, the swing arm pivot restraints on frame-contact lifts must be "capable of resisting a horizontal force of 150 pounds without permanent deformation, when applied at the end of the fully extended, unloaded arm."
The intent of the swing arm restraint is to prevent the arm from moving out of position once the arm is placed at the proper vehicle contact point and contacts that point. The swing arm pivot restraints are not intended to prevent a shifting or unbalanced vehicle from falling off the lift, as could have been inferred from the statement in the article.
Thanks to ALI for bringing this matter to our attention, and our apologies for any misconceptions the statement may have caused.