If your looking at Stant locking gas caps, you can go with P/N 10508 or 17508. Rockauto.com has them for around $6.50 and you can also find your Corolla shocks there, as well. The gas cap vents at approx. 0.5 PSI. A bad gas cap can cause the CEL to trigger due to an evaporative emissions system malfunction and, likewise, an evaporative emission system malfunction can cause the gas cap light to illuminate. The gas cap is an integral part of the evap system, which stores the fuel vapors from the gas tank until the engine can later burn them, so if it leaks or is not seated properly, the evap system detects a leak. As far as a point of failure, a lot more get replaced by people than are actually wrong. A leaking or disconnecting hose in the system can cause an evap system code to trigger, among other component failures. The vent control valve on the evap canister is a very common failure; they can stick intermittently or permanently in either the open or closed position.