you need to get yoru car to a dealership asap, a p0455 only sets after driving it for more than 20 miles and then letting it set overnight for at least 3 hours. very unlikely that it is a unproperly tightened gas cap. these are notoriously known for bad gas caps, bad filler pipes, and leaking or stuck purge solenoid valves, anytime your car in under waranty you would be an idiot to just clear the codes. You realize a p0455 means it is leaking gasoline vapors to the atmosphere at a large rate? You need to take it to a dealer before something serious happens. The other thing is disconnecting the negative cable?!?!? wtf you realize you reset all learned paramters and then ruin any fuel mileage your car should get until it has properly relearn drvign conditions and has met all mode 2, 6, 7, and 12 drive mode requirements? I am a mazda tech and i do not understand why someone would just clear the code and drive it. I will tell you right now, if it sets a code something is wrong with you car adn you need to let a professional fix it, not reset it