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Check Engine Light : Knowledge in Check

All You Need to Know About Your Engine Idiot Light

If you have ever owned a car, you've probably experienced that sinking feeling from seeing the "CHECK ENGINE" light looming large on the dashboard. Whether you refer to it as the "Malfunction Indicator Lamp," "Service Engine Soon Light" or "Idiot Light," it is important to understand the basics of this technology.

One of the most exciting improvements in the automobile industry was the addition of second-generation on-board (OBD2) diagnostics on vehicles. Adopted as part of a U.S. government mandate to lower vehicle emissions, this sophisticated program in the vehicle's main computer system is designed to detect failures in a range of systems, set a diagnostic trouble code (DTC) and turn on the "CHECK ENGINE" light if a problem is found. All 1996 and newer cars, light trucks and SUVs have this technology. Not to confuse the issue, but you should be aware that there is also OBD1 technology on many 1980s and early '90s makes and models. This first-generation OBD system monitors manufacturer-specific systems, but is not universal like the newer OBD2 technology.

Even mechanics with years of experience don't always go right to the heart of the problem on the first try.

When you turn the key, this light will usually flash for a second. If the light stays on, your vehicle may have a problem that ranges in severity from a loose gas cap, which wastes fuel and causes pollution, to an engine misfire, which could permanently damage your vehicle. In any case, there is no need to panic. It is usually just the vehicle's computer telling you to get it looked at soon.

When the light goes on, most people take their vehicle to the local mechanic or dealership. There the technician will connect a scan tool to your car through an OBD connector that is usually found under the dashboard. The machine then scans your car's computer for any DTC, which is used as a starting point to help determine necessary repairs. But remember, DTCs and trouble code definitions are only a starting point. No one — not even a qualified technician — should ever replace a part based solely on a trouble code. Even mechanics with years of experience don't always go right to the heart of the problem on the first try.

In addition to helping with repairs, OBD technology has recently become even more important. Now that OBD2 vehicle population is rising, many states are using OBD2 testing in place of the traditional tailpipe testing to pass or fail vehicle emissions or "smog" checks. As of 2004, 33 states plus the District of Columbia use this technology for I/M tests, usually required annually or every two years.

Most contemporary automotive shops, dealerships and emissions testing facilities invest thousands of dollars on their scan tool equipment and training to read complex diagnostic data. This is why they charge anywhere from $60 to $90 to "scan" your car, regardless of whether your problem is a loose gas cap or a severely damaged catalytic converter.

Just like DVD players and PCs, the longer a technology is on the market, the more affordable and widely used it becomes. Thankfully, several automotive aftermarket manufacturers have developed hand-held tools called "code readers" that are much easier to use and more cost effective than scan tools, but basically do the same thing. Code readers, which cost as little as $150, are being used by trained automotive technicians to "quick check" vehicles before and after repairs. These same code reader tools are being used by do-it-yourself mechanics and even consumers who want to be empowered before going to the mechanic, taking a long road trip or getting an emissions test.

Code readers can save you time and money by locating and identifying problems before servicing your vehicle. They let you access the same information your mechanic has — for less, to help ensure you're not getting scammed. They can help prevent future costly repairs and maintain engine performance with routine inspections. Some code readers even feature all of the I/M readiness status tests needed to see if you're ready for an emissions test. Some OBD2 code readers will even let you clear trouble codes and turn off your "CHECK ENGINE" light. But remember, if there is truly a problem that is more than just an intermittent occurrence, it will go right back on if you don't fix what's causing it.

Code Readers are great tools, but it is important to remember their limitations. Code readers only aid in monitoring electronic- and emission-related faults, so don't forget to check other parts of the vehicle such as tires, oil level, hoses and overall soundness before taking a long trip or conducting a preventative maintenance check.

If you are in the market for an automotive code reader, here are some features to look for and questions to ask:

  • Does my vehicle have OBD2? OBD2 systems, also labeled OBD II, are present on all 1996 and newer cars, light trucks and SUVS manufactured for use in the U.S., plus a few 1995 models. You can check under the hood for a Vehicle Emissions Control Information sticker to confirm.

  • Is the code reader easy to use? Look for a tool that is easy to use and doesn't require a lot of set-up, such as having to input your Vehicle Identification Number (VIN#) each time you use it.

  • Do I want a PC hook-up? Depending on your comfort level with computers, you should know that some code readers offer PC connections for additional data; others show you everything you need to know on the tool itself. Some even have a memory feature so you can see the DTC after the tool is unplugged from your vehicle.

  • Does the tool include customer support? Look for a brand that is supported by ASE Certified technicians who can answer your questions after you bring it home.

One of the easiest-to-use code readers on the market is the INNOVA® 3100 OBD2 Code Reader. You don't have to be a mechanic or computer guru to use this tool. All you do is take it out of the packaging, plug it into the under-dash connector, press a button and get all the necessary data on one screen in about 15 seconds - no set up required. If any trouble codes come up, you can access the accompanying manual to define the problem or bring the code to your mechanic for a second opinion. What sets this unit apart from other code readers is its ease-of-use, patented single-screen display, speed, memory function and affordability. Sold at more than 5,000 retail and automotive stores throughout North America, this tool is used by everyone from professional mechanics and do-it-yourselfers to consumers and even the California Air Resources Board (CARB) to test vehicles.

Regardless of whether you invest in a code reader or drive straight to your mechanic, be sure to pay attention to that "CHECK ENGINE" light. A little education and preventative maintenance can go a long way when it comes to preserving the life of your vehicle and your blood pressure when the "idiot light" looms large.

Established in 1982, the company's Equus®- and INNOVA®-branded test equipment and gauge products are available at most North American automotive and mass merchandise retail locations. For information, visit

Some additional "Check Engine" light resources: and