Ten percent of us have a "check-engine" light illuminating our dashboards right now, and half of us have been ignoring that light for the past three months. According to the automotive research firm CarMD.com, the ramifications of our laissez-faire attitudes toward that little yellow light can range from minor (a $73 adjustment to the transmission shift control cable) to catastrophic (a brand new inverter assembly on a hybrid vehicle, for $4,098). One thing is almost always the case: Regardless of the underlying problem, if the check-engine light is on, your fuel economy suffers. 

Today, CarMD.com released its second annual Vehicle Health Index Report, detailing the most common check-engine light problems, the related repairs and the cost of those repairs. The good news is that in 2011, there was a decrease in average national auto -epair costs. But CarMD.com's data also showed that people still put off small fixes that can have big ramifications down the line, and that can hurt fuel efficiency. With gas prices so high that you're best off keeping your eyes shut at the pump, the company highlighted the five most common repairs that affect fuel efficiency. 

At the top of the list is the need to replace the oxygen sensor, which can reduce gas mileage by up to 40 percent if ignored. Second in line is one of the simplest car repairs of all: replacing a loose or missing gas cap. It's a problem that accounts for 147 million gallons of evaporated gas each year and can hurt your gas mileage by 0.5 percent, according to CarMD.com. 

Replacing the catalytic converter is the third most significant car repair vis-a-vis gas mileage. A damaged catalytic converter will gradually degrade your fuel economy and could eventually kill your car altogether. The fourth most common repair for preserving fuel economy is fixing the ignition coil; if ignored, it can drain your mileage by 20 percent. Fifth on CarMD.com's list is the mass air flow sensor; your fuel efficiency will suffer by 10 to 25 percent if you continue to drive around when it needs replacing.

"Our goal is to remind drivers that there are many small, cost-effective ways to save money even as gas prices reach an all-time high," said Art Jacobsen, vice president of CarMD.com. "In many cases, the repairs will pay for themselves very quickly in fuel savings."

CarMD.com also has several general tips for maximizing fuel economy. It recommends against skipping scheduled maintenance appointments; doing so could be more costly in the long run. The company also encourages you to rid your vehicle of clutter; the lighter your load, the better your mileage. And according to CarMD.com's research, idling your car for more than 10 seconds burns more gas and creates more pollution than restarting it. So you're better off killing the engine than idling. Lastly, pay attention to your tires; proper tread and inflation can increase your mileage by up to 10 percent.