BOSTON (TheStreet) -- Some 10% of U.S. vehicles have their "Check Engine" lights on at any given time -- but half of drivers ignore the warning because they fear their cars will need expensive repairs, market watcher has found.

"People say: 'My car is driving fine' or 'This is just my dealer trying to stick it to me,'" CarMD's Kristin Brocoff says. "But if you ignore the problem, it's just going to snowball."

All U.S. cars built after the 1995 model year come with on-board diagnostic computers that constantly monitor some 80% of a vehicle's systems to uncover problems as soon as issues develop.

Mandated originally by the U.S. government to spot malfunctions that increase your car's emissions, many on-board computers now go beyond the official requirements and monitor a vehicle's battery, antilock brakes and other systems.

Serious issues will prompt the "Check Engine" light to illuminate on a car's dashboard -- something many vehicle owners dread. After all, you'll need to take your vehicle to a repair shop or buy a special handheld device from CarMD or one of its competitors just to find out what's wrong.

Many of the roughly 650 problems that can trigger the "Check Engine" indicator also cost big bucks to repair. For instance, CarMD says a broken transmission assembly -- the most-expensive problem associated with the warning light -- runs about $5,474 to fix.

Fortunately, CarMD says serious problems represent only 1% of cases involving "Check Engine" lights.

The firm, which analyzed 161,000 indicator-related repairs done last year by U.S. mechanics, found that the average malfunction costs just $368 to fix.

That's good news, because your car will fail annual safety-and-emissions inspections automatically in many states if the "Check Engine" light is on. Some problems will also slash your vehicle's fuel efficiency, Brocoff adds.

Here's a look at the five most common things CarMD found triggered "Check Engine" lights last year, as well as a rundown of average repair costs:

Broken oxygen sensor
Share of all 'Check Engine' warnings: 8.3%
This problem is cheap to fix but costly to ignore, as a malfunctioning sensor can cut your car's miles-per-gallon rating by up to 40%. That can boost the typical driver's annual gas bill by as much as $900, according to CarMD.

Fortunately, the firm estimates replacing a busted oxygen sensor will set you back only $294.

Loose or damaged gas cap
Share of all 'Check Engine' warnings: 7.2%
Resolving this issue costs only 10 cents on average because most mechanics will fix it for free. After all, the "repair" frequently involves nothing more than tightening your car's gas cap.

If the cap is missing or damaged, garages charge just $22 on average to replace it. Or you can buy one at an auto-parts store for less than $10.

Broken catalytic converter
Share of all 'Check Engine' warnings: 6.3%
Replacing a catalytic converter averages a hefty $1,029, but Brocoff calls this problem "the perfect example of something that you can prevent for 10 bucks if you don't ignore it for too long."

She says many catalytic converters fail due to defective spark plugs -- a malfunction that will trigger your car's "Check Engine" light all by itself. Change the plugs when the indicator first comes on and your catalytic converter should keep working fine.

Damaged ignition coil(s) and spark plug(s)
Share of all 'Check Engine' warnings: 5.3%
This is another issue you can avoid by quickly addressing a "Check Engine" light caused by old spark plugs.

Bad plugs will eventually damage not only the catalytic converter, but also the car's ignition coil or coils. Expect to pay about $317 if the latter happens.

Worn spark plug(s) and wiring
Share of all 'Check Engine' warnings: 3.5%
As noted above, keeping your spark plugs in good condition will avoid bigger problems down the road.

Do-it-yourselfers can typically replace a single spark plug for less than $10, or a mechanic can swap them all out for around $343 (including related wiring).