You must have disconnected an airbag before the electrical system was totally discharged or before the battery was disconnected. In order to prevent the airbag diagnostic unit from setting a check light the following must be done before disconnecting any airbag wiring:

1. Disconnect the battery.

2. Turn on the headlights.

3. Wait approx 15 minutes.

If this procedure wasn't followed the airbag system will indicate a fault. The only way to reset the system is with a Kia Pro Scan Tool. And only Kia Dealers have this tool so unfortunately you must visit a dealer and have the system reset. And expect to be charged a diagnostic fee.

If a new gas cap didn't fix the P0440 code then inspect all the vacuum lines going to the vapor canister unit.