To Fahrion, that's the real power of OBD II information. "If you can diagnose a misfire—which is usually a sporadic event that happens under certain load conditions and other variables—this gives the technician a lot of guidance," he says. "It says, 'Look, this is the cylinder,' and from there all you have to do is decide whether it's a mechanical problem, like a loss of compression, or a fuel problem or a spark problem."
Fahrion explains that the third digit points the technician to the circuit or system of the vehicle that's involved. A '1' or '2' both point to fuel and air-metering-related problems. A '3' points to an ignition system problem, '4' points to auxiliary emissions, '5' to speed and idle control, '6' to computer output, and '7' and '8' to the transmission. The last two digits refer to the specific fault, with a total 99 combinations. "Once you boil it down to those basics," he says, "it takes a lot of the mystery out of it."
Or at least it should. Unfortunately, OBD II information overload remains a problem for some technicians.
"It's a grand search for information, and putting pieces together," says Menchu. "You get a little data here, a little data there. You use the scan tool to get a feel for the system and make a conclusion, and then you try to confirm it and make a decision on it, that's absolutely overwhelming."
Menchu, who pioneered the automotive lab scope and now conducts diagnostics seminars around the country, feels that technicians can always work smarter, no matter what diagnostic tool they use: "You can have all the information in the world, but if you don't have a good personal foundation, it's not going to do you any good."
"If you look at this from the technician's perspective, what's key in the utilization of that OBD II information is to really understand what the intent of that information is," echoes Diego Borrega, director of product engineering for Networkcar. "All those OBD codes come as a function of these tests being run."
"One of the things I see as a trap is, there are a few problems that technicians immediately associate with a certain code," says Borrega. "Say you have a 1999 Chevy pickup with an eight-cylinder engine. The technician knows from experience that every time he sees a P0004 code on that truck, it means this particular sensor needs to be replaced. Now, that's a connection they've made in their heads, but that's not necessarily the significance of that code. The significance of that code is that a certain test has been run, and a certain value is out of specification. So, from a technician's perspective, the information is most valuable when they understand how that information came to be.
"So, one of the pitfalls is to think that, just because you recognize the code, you know exactly what's wrong with the vehicle," he says. "You still have to understand the diagnostic trees, and understand how those codes came to be and the fact that there was a particular test run behind that code. That's why it would be a mistake to look at the most common fault codes and decide that it means this particular component is failing most often. All of these data are meant to help, but you need to understand the context of that code in order to be able to extract useful information from it.
"In general, you can't really tell what's broken by one code," he stresses. "One code tells you that a certain parameter is out of spec'. For example, a catalyst code says that one of the monitors inside the vehicle to test the catalytic system has found a certain voltage out of spec'. Does that mean the sensor's broken? It could just mean that the sensor that's detecting that voltage is broken. It could mean there's a broken wire. Or it could mean that the vehicle is truly polluting."
AutoTap, the diagnostic tool offered by Fahrion's company, is popular among do-it-yourselfers, but it also appeals to technicians. Because it's a PC-based tool, Fahrion explains, it can manage information in a way that helps the technician be more productive.
"A hand-held scanner is going to look like a volt-meter, and it's going to have a one to four-line display on it, and a handful of buttons," he says. "You point and shoot, plug it in and it tells you what the code is. The more money you spend on the scanner, the more buttons you can manipulate to get you real-time data."
Dave Cappert examines the most important scan tool functions.
Not long ago, the scan tool was simply one of the tools you could use to help diagnose emissions problems. My, how times change.
OBD II changed maintenance forever--OBD III is set to do the same.
Technical Editor Dave Cappert answers your questions about scan tools.