same problem on my wife's tc. quickly found the same gas cap problem. it clicks but wont seal. It comes off easily with zero resistance. I swapped caps with my tacoma, but NO FIX. When properly locked in place, the cap needs to provide enough resistance, during opening, to recoil the spring inside the cap. The problem must be the actual fill neck (which is what the dealership quoted on the OP's initial post.)

Anyone try replacing the fill neck? or try to tweak it/bend it on a bench?

 

i got new gas cap.. reset the battery. the cel went away. came back next day.
got the gas cap from toyota. dont know what to do

 

I just replaced my defective / loose gas cap earlier from toyota shop for $23 but the check engine warning still lit up. I back read this thread and someone mentioned about waiting 2-3 days til the warning goes off. I'll try to do the same but if it still persists, I'll bring it back to toyota service for them to diagnose or reset the CEL.

 

I replace mine gas cap, but my car still stalls out

 

My 2006 tC with 33000 miles failed the WI emissions test because any Check Engine code is an automatic fail. The printout showed the P0456 code which led me to this thread. I bought a new OEM gas cap off Amazon for $20 and my CEL light went out after 3 days. I didn't do the battery disconnect or the fuse reset it just went out by itself. Passed the emissions test once the light was off.