Gas caps have been around forever as a way to keep fuel from spilling out, contaminants from getting in and to minimize evaporation. Most were just simple metal caps but some came with vent holes to equalize the pressure with the atmosphere. A gas tank is like any liquid container, as liquid is taken out air must be allowed in to take its place.
On some older vehicles that had vented caps a plugged vent or vent hose could actually prevent the car from running. Or the wrong replacement cap was installed which didn’t have the required venting. If air is restricted, the fuel pump is pulling against a vacuum and may eventually get no fuel.
But the modern fuel cap is now an integral part of the emissions system. It may have specific design requirements depending upon the exact make, model and even engine installed. Since the 1970s the emissions laws have all but eliminated the original simple gas cap. Replacement caps need to be exact equivalents to the original design to prevent system error codes.
If you’ve ever gotten a CEL (Check Engine Light) with error code P0440 or P0442 it most likely was just a loose gas cap. When an OBDII vehicle (1996 and later) is started it runs a series of system tests throughout the vehicle checking that all systems are operating correctly within programmed parameters. A loose fitting, poorly sealed or missing gas cap will be detected and will set off an error code.
The Check Engine warning light comes on to warn you to have a problem investigated. If you’ve been driving with no CEL issues and after stopping to fill up your fuel tank find that you get this warning light when you drive away, stop and check your cap. The warning may not come on immediately after start up but a few miles later depending on your vehicles age and testing sequences. Whenever you fill up the tank reinstall the cap until it clicks (if cap has that feature) which indicates it’s fully seated on the filler pipe. A loose, incorrect or missing gas cap is one of the most common CEL problems.
On some newer models now a capless filler may be found which has no gas cap at all. These operate with a sealing flap that the gas nozzle opens when you insert it at the filling station. This eliminates the potential for losing your cap or leaving it on the filling station pump. But it also means you need to be careful keeping the filler door closed to keep the filler area clean.