What your car's 'check engine' light is saying

Emission control is a touchy process that involves sophisticated sensors that compare the oxygen entering and leaving my "cat."

Because so many other parts of our engines affect what ultimately comes out of our tailpipes, it takes an OBD system to keep everything performing properly.

In 1996, the feds required U.S. automakers to upgrade to OBD II, a sophisticated system of sensors and actuators that conduct on-the-road diagnostic tests. When the OBD II detects a problem, it logs it as a trouble code in its computer memory. Once retrieved via a diagnostic reader, this code directs the repair technician to the source of the trouble.

While OBD II set the standard for American-built passenger vehicles, many foreign automakers have piggybacked their own proprietary diagnostics onto it. As a result, the check engine lights themselves (and the range of well more than 100 possible problems they can detect) vary considerably.

On some cars, the light simply goes on. On others, it may flash red to indicate an immediate, potentially damaging situation or simply illuminate for non-emergency problems. Still others will display yellow for non-emergency, emissions-related glitches and red for stop-worthy situations.

There are two ways to turn off the check engine light.

Here's what Ben Moreno, service manager at Southwest Auto in Dallas advises: "If your check engine light comes on and you have no symptoms -- the car's running fine, your temperatures are normal, it's not hard to start -- then schedule to have it looked at.

"If you do have a symptom -- it's hard to start, slow to accelerate, there's black smoke -- let's get it in."

Bright light, big bill
About now, you're probably wondering how far my check engine light might put you in the red, right?

Well, there's good news and bad news.

First, the good news. My OBD II system is so sensitive that in many cases the problem is a simple human error that can be remedied for free.

"Eighty percent of the time, it's nothing more than people forgetting to tighten the gas cap," according to Gary Martin, owner of Martin Motorsports Inc. in Alexandria, Va.

"Or people forget to put their dipstick in tight. The car doesn't run that well because it loses vacuum, it starts to idle funny, the computer senses it and boom -- a code."

A tank of bad gas also can trip a code. So can filling my tank while the engine is running, never a good idea anyway.

Although some dealers and repair shops charge upwards of $75 to hook me up to their diagnostic reader, others will read my OBD II and clear the fault (i.e., turn off the check engine light) at no cost. Just call and ask.

And, all things considered, it's generally cost-effective to repair a minor problem before it becomes a major one.

Now the bad news.

If my trouble code is not due to driver error, Moreno and Martin say that nine times out of 10 it will be traced to an emissions problem, typically either a perforated vacuum hose (repair cost: $100 to $200) or failed O2 sensors ($200 to $600 to replace the pair). Replacing airflow sensors will run in the neighborhood of $400 to $900 depending on the make. A new catalytic converter can run anywhere from $400 to $1,500.

If my code is transmission-related, often a flush-and-fill transmission service ($150 to $200) will solve the problem. But if my tranny is shot, figure on dropping $1,500 to $3,500 for a new one.

Worst case, if my check engine turns out to be a "chuck engine" light, you're looking at anywhere from $2,000 to $8,000. There are antacids in my glove box.

On the bright side, Martin says sometimes the check engine light can actually save you money. That's because some automakers program scheduled maintenance into the ODB II that will trip a code at a specified time or mileage.

"If you're replacing a timing belt and from the readout you know you're going to have to do a water pump soon, why not do it all at the same time?" he says. "You've got to take the same stuff off anyway. Why pay for the same job twice?"

What about the bulb?
With all that responsibility riding on my check engine light, how do you know the light itself isn't burned out?

That's easy. When you turn my key to the "on" position, before starting the engine, all of my warning lights should come on. Once you start my engine, they should all disappear, unless there's a trouble code, of course.

Knowing that procedure can mean the difference between picking a winner and plucking a lemon when purchasing a used car, says Moreno.

"You want to make sure all the indicator lights light up because some people, rather than fix their car, will remove the bulb and sell the car," he says. "You wouldn't have anything there to let you know that a symptom was hiding."

If you want to make extra-sure that the used car you're thinking of buying is in good health, have an auto shop scan its OBD II code log; many OBD II systems keep a record of the car's codes in memory.

Or you can buy your own ODB II handheld code reader from any number of vendors for under $200. But because readers have become increasingly manufacturer specific as automakers pack their OBD II systems with proprietary codes, a nonspecific reader may not tell you much about a proprietary code.

Now let me give you some tips on my satellite navigation system while we drive to the repair shop.

Jay MacDonald is a contributing editor based in Mississippi.