Rose Horton, checked your code and in most cases it is the gas cap, if you have replaced that and it is still not working still I would suggest you look for a leak.
and more from here
P0455 Evaporative emissions gross leak detected
1) Fuel filler cap missing or loose.
2) Fuel filler cap o-ring damaged or missing.
3) Faulty or damaged fuel filler pipe.
4) Leaking fuel vapor lines.
5) Improperly installed or leaking Purge Solenoid Valve (PSV).
6) Leaking canister or catch tank.
7) Leak Detection Pump (LDP) body leaking.
1 Check fuel filler cap for being tightly installed, has o-ring seal installed and is in good condition. Verify cap releases pressure/vacuum at specified values (approximately 2 psi pressure and approximately 1.5 inches of mercury vacuum).
Are cap, o-ring and release pressures okay?
YES Go to step 2.
NO Replace fuel filler cap.
2 Check fuel filler pipe for cracks, pinholes, damage and o-ring seat for deformation.
Is fuel filler pipe okay?
YES Go to step 3.
NO Replace fuel filler pipe.
3 Per diagram "A", thoroughly check all fuel vapor hoses and hose clamps between:
1) Canister and rollover / ORVR valve.
2) Canister and LDP.
3) Canister and PSV.
4) 4-way Distribution Manifold and fuel filler pipe.
5) PSV and intake manifold. (Continued on next page)
(At this point, verify arrow on PSV is pointing towards intake manifold. If it is not, reverse installation.)
Are vapor hoses and clamps okay?
YES Go to step 4.
NO Replace ALL cracked / damaged hoses or loose clamps.
4 Turn ignition off, disconnect C254 from ECM and install adapter cable K99U-2106-G17 to BOB. Connect adapter cable C254 to ECM and harness C254 to adapter cable. Disconnect hose leading from PSV to intake manifold at PSV. Per diagram "B", draw a vacuum at nipple "B" with a mityvac (do not exceed 20" of vacuum) and verify PSV holds vacuum. Turn ignition on and jumper a wire from BOB pin 36 to GND (should hear a faint click from PSV). Vacuum should bleed off. Repeat this procedure 4 or 5 times to ensure PSV reliability.
Is PSV working properly?
YES Go to step 5.
NO Replace PSV and then go to step 5.
5 Return PSV hose to original condition. Disconnect C154 from PSV and prepare multimeter for current draw using 300 milliamp scale. Attach negative probe to chassis ground and positive probe to BOB pin 20.
1) Turn ignition on (engine off) and record LDP current draw (allow 5-10 seconds for current to stabilize). Minimum 17mA, maximum 36mA.
2) Jumper BOB pin 18 to chassis ground and observe current draw of LDP. Within 5 minutes, current draw should be greater than or equal to reference draw recorded in step 5a.
Was LDP current draw in step 5b greater or equal to current draw in step 5a?
YES Go to step 6.
NO Using appropriate evaporative emissions leak testing equipment, (or redo step 5 using an R-134A sniffer or exhaust gas analyzer) locate source of leak and repair as necessary.
6 Clear codes and return vehicle to original condition. Verify any repairs by driving vehicle with KIA Data Pro connected to OBD-II connector and monitoring for pending codes (refer to section 3 of the KIA Data Pro Generic OBD-II Program Card reference manual).
Lets hope its just the gas cap ;-) good luck and I hope this helps.
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