Driveability Corner Will a scan tool’s sample rate and the PCM’s reporting capabilities allow you to reliably test a conventional zirconia dioxide oxygen sensor? The answer depends on when and how you run your tests. Mark Warren smwarren@motor.com I s a generic scan tool sample rate fast enough to test a conventional zirconia dioxide oxygen sensor? Let’s take a look to find out. Fig. 1 is a recording of bank 1 sensor 1 on a 2000 Honda Odyssey with a 3.5L engine. The recording was set up to capture just the bank 1 sensor and no other parameters. During this recording I was holding the throttle at about 2000 rpm (80 to 100 seconds on the scale at the top of graph), then snapped the throttle wide open and shut (100 to 110 seconds). I repeated the same procedure between 115 and 140 seconds. When snap-testing an O 2 sensor, what data is important? •Minimum and maximum voltage. •Rich-to-lean and lean-to-rich transition times. •Does the O 2 sensor exhibit negative voltage? If it does, how much? •What is the switch frequency at 2000 rpm? •Is the scan tool’s sample rate sufficient? Fig. 2 is a zoom in on Fig. 1. You can see that the green cursor is in the same location in both figures, at the 100.8-second point. Fig. 2 is configured to show the actual data points, with graph lines drawn in between the data points. Looking at Fig. 2, let’s answer the O 2 sensor test questions. Minimum voltage is zero. When testing an O 2 sensor with a continued on page 18 Fig. 2 Graphs: Mark Warren Fig. 1 16 June 2012 Driveability Corner Fig. 3 snap test, voltage must drop below .200 or the sensor must be replaced. Maximum voltage is .940 volt; the sensor must go above .800 volt or be replaced. The rich-to-lean and lean-to-rich transition times are measured from .800 to .200 and from .200 to .800 volt. The transition time on a good sensor is less than 150 milliseconds (mS), or .150 second. Note that the time in seconds at the top of the Circle #15 18 June 2012 graph is .4 second per major division. The switch times from rich to lean and lean to rich are reasonably close to 100 to 150mS. Does this O2 sensor exhibit negative voltage? Okay, most vehicle PCMs do not report negative voltages to the scan tool, but some do. The fact that the O 2 sensor pegs on zero for about 2 seconds on the lean transition leads me to believe that some negative voltage may be present. How much is too much, and how do I know? The best testing method is with a scope, because sample rates are way better and negative voltage is easy to quantify. But we’re trying to get the fastest, easiest test using a scan tool here. Sensor makers and vehicle manufacturers usually specify that no negative voltage is acceptable. However, I’ve seen negative voltages on brand-new vehicles. Using Fig. 2 and counting the ple rate is fast enough for this type Experience shows that up to sample points that occur in 10 secof O2 sensor analysis. 150mV negative is okay but below Fig. 3 on the previous page onds, we can see that we’re getting that, replacement is in order. As shows a graph of a similar capture about seven samples per second, or oxygen sensors shift negative, the on the same vehicle while recordabout 150mS between samples. maximum voltage ing 10 data parameters. generally drops proI put up rpm as well p o r t i o n a l l y. T h e r e so you could see the fore, when very negaThe best testing method is with snap-throttle events. tive at the minimum Other than min/max a scope, because sample rates voltage, the sensor voltages, no other oxywill usually not gen sensor analysis is are much better and negative achieve the required possible because the max voltage. additional parameters voltage is easy to quantify. The switch frehave slowed the samquency is about once ple rate. Also, note the angular nature of the per second, or 1Hz. graph; when the sample rate is too We have a sufficient data sample This is just enough to get a pretty rate to analyze the switch rate. slow, this is a good tip-off that samgood idea about transition times. Switch rate is a function of O2 senpling is very slow. Remember, this is the ISO commuKnowing your sample rate and sor response combined with the nication OBD II protocol. Some the speed of your signal will make fuel system’s response rate. Note may be slower and some may be that the rate is slower at idle speed diagnostics easier and help you faster, and you’ll have to look at avoid a misdiagnosis. and is rpm-dependent. the data to determine if your sam- MACHINE IS THE PARTS. A SUM OF ITS Ever wonder what makes a Power Stroke® Diesel engine so dependable? It’s the combined effort of high-quality components that are specially designed to work with each other to ensure long engine life and achieve impressive results. Don’t mess with that. Demand OE parts. Using OE-recommended parts for repair makes sense. They ﬁt right the ﬁrst time, help reduce vehicle downtime, and are the only parts that come recommended by Ford Motor Company. With a nationwide warranty* that is backed by more than 3,000 Ford and Lincoln Dealerships, go with what got you here. *See dealer for limited-warranty details. Visit your local Ford Dealer or FordParts.com to purchase and see PowerStrokeDiesel.com for additional product information. GENUINE POWER. GENUINE PERFORMANCE. GENUINE PARTS. Circle #16 June 2012 19