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It was enough to make Melissa Thom’s heart sink.

She was driving to a Christmas Day family get-together that was about 200 miles away. Along the way, she noticed the “check engine” symbol was on the instrument panel.

“Of all the days this could happen,” she said.

Figuring there wasn’t much chance of having the car checked, she checked that the oil was full and kept going. The car ran fine, so she also drove home and will now see about getting it fixed.

But she wonders: Could she have damaged anything? Was this a bad thing to do? How can you know what’s wrong?

By and large, Thom made it through a stressful situation and managed it well.

Her illuminated “check engine” light indicates an emissions-related fault is occurring, and the possible reasons could be many. Unlike the red oil and temperature warning lamps that require immediate attention, the amber “check engine” light carries a softer message. In some cases, a steadily illuminated lamp refers to a situation that could affect engine performance, while in the majority of cases the engine management system is able to work around the fault.

Had the lamp been flashing, indicating a severe engine misfire, I would have turned back and made other plans. Such a fault can cause a large lack of engine power, produce huge exhaust emissions, and probably destroy your catalytic converter.

When confronted with a glowing “check engine” light during a trip, I’d size up the situation by stopping somewhere that’s safe and convenient and walk around the car. Listen to and smell the exhaust. If the sound is consistent (not thumping) and is odor-free, then so far, so good.

Next, give the gas cap a firm tightening twist and sneak a peek beneath the front of the car, looking for leaking coolant. Checking the oil is never a bad thing to do, but stay away from the radiator cap if the engine is warm/hot. These checks are unlikely to have a lot to do with the “check engine” light, but are prudent, as well as confidence-builders.

Does the engine restart promptly and accelerate smoothly? If so, I’d hit the road and get where I need to go. If climbing hills or driving in hot weather, listen for pinging, a jingling noise from the engine. If pinging is heard, try to mitigate the condition by easing up on the throttle/slowing down.

The light may be trying to tell you your catalytic converter has seen better days, your gas cap was loose, or a faulty sensor is being ignored and replaced with inferred information. I’d certainly have someone pull codes – that is, connect a scan tool and retrieve stored diagnostic data – at the first convenient occasion. A trouble code doesn’t indicate the exact cause of a problem, but at least provides information about the neighborhood and severity of the fault.

Here’s a neat project for smartphone enthusiasts: Go to eBay and type in “elm 327”. You’ll see Bluetooth OBD-II scan tool interface devices for less than $20. Purchase one, then search your mobile application store for “OBD DroidScan,” “OBD Auto Doctor” or similar. These inexpensive apps, coupled with your interface device, provide code reading, code clearing and limited engine data. Expect a little connectivity hair-pulling, but it’s worth it.