I've had my resonator out for like...4 months now and I have an HKS Hi-Power. There's no way your car can detect something like that. The last sensor the Celica has is just after the Catalytic Converter. If you removed the resonator then that sensor should still be intact and working fine because it cannot detect anything after that. I just don't believe you can get a CEL from removing the resonator, you're the first I've heard of for sure.
I agree! Think about the people with the Kazuma exhaust, wouldn't they have even less back pressure not have any mufflers, canisters or resonators? And no one I know with the Kazuma had any problems. Could it be anything else that's causing it?