Is the gas cap causing my car’s warning lights to go on?

John Paul, AAA Northeast's Car Doctor, answers a question from a reader with concerns about persistent warning lights.

AP Photo/Lynne Sladky, File

Q. In February I purchased a 2006 Lexus SC 430 with just over 50,000 miles. My problem is with the vehicle stability control light and check engine light that keep popping up on the dash. Online info indicated it’s a gas cap problem and to just clean it. I’ve done that, but the light comes back on after a while. Is there a special way to clean the cap?

A. The first step would have any diagnostic codes read to determine possible problems of both lights. The lights could be triggered for a single issue or multiple problems. The vehicle stability control light typically will illuminate if there is an issue with a wheel sensor. When the VSC light is on, the system shuts down, and although the car will behave normally, you’ll lose the skid prevention aspect of the car. The typical check engine light issue, although it could be related to the gas cap, is more than likely an issue with the evaporative emissions system (the gas cap is part of that system). The light is usually triggered by a faulty purge control valve. This system is best checked with a “smoke-test.” Synthetic smoke is pumped into the evaporative system and the technician looks for smoke leaking from various components. 


Q. I have a 2005 Toyota Corolla with 190,000 miles. The air conditioning has always worked extremely well until now. When I am stopped or driving slowly in stop-and-go traffic, it blows lukewarm air. If I’m driving at regular highway speeds, it works fine. I will mention that this past year I did have repairs to the car in the way of a new radiator and flush as well as a transmission flush and starter replacement. I cannot imagine that any of these repairs would have affected the efficiency of the AC. What do you think may be the problem?

A. It is unlikely that the work performed would have caused any issues with the air conditioner. More than likely there is a slight leak in the air conditioner system and the refrigerant level is low. At 17 years old, it’s not unusual for seals to start to dry out and seep refrigerant, resulting in a low charge. At this point the best thing to do is have the air conditioner tuned up. The technician will look for leaks, test AC performance, and check refrigerant levels. If the level is low, the technician may add some refrigerant with both a dye (to check for leaks) and a sealer which may prevent or slow minor leaks. 


Q. For an electric car, why can’t the motor spin a belt that turns a generator that charges the battery? In gas powered vehicles we have an alternator. Why not use it or a similar device in electric vehicles?

A. The simple answer is that the car would need a very large generator, and the energy needed to turn the generator under load would still take more out of the battery than it could put back in. Electric vehicles and plugin hybrids do use regenerative braking which does add some electricity back to the battery, but not enough to fully charge the battery, which is why these vehicles need to be plugged in. BMW’s electric i3 model can be equipped with a small gasoline engine that powers a generator. This system maintains the charge level of the high-voltage battery, so the car can run on electricity. Ford with its F-150 hybrid pickup truck has an onboard generator that can produce 240 volts at up to 7.2 kilowatts. This would be enough to charge an electric car, but it is powered by a turbocharged V-6 engine. A startup vehicle manufacturer Aptera is promising an electric vehicle that can charge up to 40 miles of range, using the sun. For many drivers this may eliminate plugging in.


John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE-certified master technician. E-mail your car question to [email protected]. Listen to the Car Doctor podcast at


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