But that's rationalizing, of course, and potentially dangerous to your wallet. Brocoff says the most common reason a check-engine light is on is due to a faulty oxygen sensor, which can reduce your miles per gallon by 40 percent.
"And the worst part?" says Brocoff. "Not only will you pay to put extra gas in your car, you'll still have to eventually fix the problem."
And how much will it cost? If you know how to fix it yourself, you can buy a replacement oxygen sensor for about the price of a tank of gas. If you need a mechanic, of course, you'll need to budget for several tanks, at least.