Check Engine Light: Scan Tools and Testing

How diagnostic tools identify troubling symptoms on your car

You're purring down the highway, not a care in the world and the Check Engine light comes on. Now what? Do you pull over to call for a tow or see the glass as half-full and keep going? It's a little like having a doctor peer over your files and say, "You have a problem." Is the solution a good night's rest and more fiber or is it time to update your will? In reality, that Check Engine light could mean anything from a defective light to the automotive equivalent of major surgery.

Self-Diagnose Your Car

The good news: You can get a diagnosis without heading to your mechanic. The auto aftermarket's answer to the labyrinth of microprocessors and engine control units (ECU) is a selection of stand-alone or laptop compatible diagnostic tools so you can determine and, hopefully, resolve the problem without the hassle of an overnight in the mechanic's bay.

All this technological wizardry began back in the 1970s when the Environmental Protection Agency (EPA) established strict emission standards. The state of California compounded the issue with even stricter regulations, forcing the auto manufacturers to comply if they wanted to sell cars in the world's most car-obsessed state. The solution to controlling emissions was simple: Just control the air/fuel mix with computer technology. The ECU, loaded with optimum engine parameters, monitors data from the engine's sensors and adjusts everything from spark timing to how long the fuel injector is open. Where appropriate, this info is communicated to your vehicle's instrument panel, including that pesky little Check Engine light.

As long as the engineers were computerizing our engines, they figured it would be relatively simple to use all this data to help diagnose engine issues. On Board Diagnostics (OBD) have evolved since the early 1980s from non-standardized systems to the current OBDII standards, mandated for all cars sold in the U.S. beginning with the 1996 model year.

Plug and Play

Diagnostic tools are simply plugged into the port (usually located on the driver's side of the passenger compartment near the center console) and out pours information on what ails your vehicle. Of course, the diagnosis information is in code, but the value of OBD is obvious. The mystery of the engine light can now be solved. Plus, those annoying intermittent problems that disappear the minute you pull into your mechanic's driveway and then re-appear after you leave cannot hide from the diagnostic tool.

Diagnostic tools fall into a couple of categories, ranging from those professional systems used by mechanics to consumer units, some of which require a laptop, others simple stand-alone tools. Before the advent of computer-controlled engines, auto enthusiasts could troubleshoot engine problems and tweak carburetors for improved performance. Those do-it-yourself repairs and adjustments limited only by the size of the enthusiast's garage and tool kit.

Today's scan tools won't return us to those blissfully simple days, but they give that frustrated enthusiast a valid picture of how the engine is running. Armed with that information, you can determine if you're up to the repairs or approach the mechanic's bay with your own diagnosis. Weekend racers also use these tools to analyze and optimize their car's performance.

Laptop or Handheld

The consumer tools that require a laptop computer are less expensive, with nearly unlimited storage capacity for data logging and other functions. The screen offers higher resolution, you've got a choice of software programs and the more sophisticated units can actually reprogram some of the ECU's parameters. On the down side, you have to have a PC laptop to do your diagnosis.

The standalone tools are available in a broad range of functions from simple code read tools to more sophisticated functions. They are easier to operate and require little to no actual computer skills. Many come with carrying cases and offer rugged construction with the intent of living in the vehicle's glove compartment or storage bins.

If all this whets your appetite to communicate with the mystery that is your engine, here are a few features to look for in standalone scan tools: Make sure the OBD codes are compatible with your vehicle. The codes should include both generic and manufacturer-specific codes. If your vehicle is pre-OBDII vintage (before 1996), can the tool be upgraded with the appropriate codes?

Along those lines, is the tool updatable? Units that last your lifetime, rather than your vehicle's, should be updatable over the Internet. Can you read the screen? The tool should not only display the trouble code, but define it.

All told, maybe the most important feature, it should be able to reset that puzzling Check Engine light.

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