Defective Gas Caps Can Lead to Failed Emissions Test

The initial use of a gas cap was to keep dirt and water out of the fuel tank. However, as automobiles got more sophisticated, the function of the gas cap increased. Over time, it has become an emissions control device.

Gas_Cap.pngOne element of the Colorado vehicle emissions inspection is the Evaporative Emission Control System (EVAP) check. An automobile EVAP prevents vapor from the fuel system and gas tank from being released into the atmosphere. One reason for an emissions EVAP system failure is a defective/missing gas caps.

The Car Care Council states that, "A missing or leaking gas cap can allow up to 30 gallons of gasoline per year to evaporate into the atmosphere." Using today’s gas prices, a bad cap could cost you more than $100 a year. In addition, these escaping gas fumes, when combined with heat and sunlight, cause the type of harmful air pollution known as ground-level ozone.

Unlike the good, protective ozone layer in the stratosphere, ground level ozone is a harmful air pollutant that affects all of us. It’s formed when emissions from everyday items combine with other pollutants and “cook” in the heat and sunlight. Weather plays a key role in ozone formation. The highest ozone levels are usually recorded in summer months when temperatures approach the high 80s and 90s and the wind is stagnant of light.

Colorado Puts a Cap on Ozone

If you are due for a vehicle emissions test, here is a reminder to make sure your gas cap is in place and secured tightly to give you the best chance to prevent a failure. However, from July 1 through Aug. 31 , 2013, any vehicle that fails the Colorado emissions test due to a faulty or missing gas cap will be provided with a voucher good for a new cap redeemable at NAPA Auto Parts stores located throughout the Greater Denver area and the North Front Range.

All failed vehicles must be retested. Retesting is FREE if completed within 10 calendar days.

For more information on solving the ozone problem, call the Regional Air Quality Council at 303-629-5450, or visit

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