P0546 - Exhaust Gas Temperature Sensor Circuit Malfunction (High Input)
This code happens most often when you replace a with-catalytic-convert up pipe with a catless up pipe. The result is that the exhaust gas temperature (EGT) sensor that goes in that pipe is exposed to a blast of flaming exhaust gas, and eventually burns out. Replace the bad EGT sensor, and the new one will probably burn out as well. To avoid having to keep doing this, you can just remove the sensor -- its whole purpose in life was to protect that catalytic converter that's no longer there -- and put a resistor in place of the sensor to make sure the ECU stays happy.
Specifically, you want to remove the EGT probe from the up pipe, and plug the hole. If your up pipe did not come with a plug, you'll need a short M12 x 1.25 bolt, like an oil drain bolt. Use high-temp thread lock to make sure the bolt doesn't back out.
Remove the probe from its wiring harness, and insert a 2.2 K Ohm resistor, available at any radio shack. There are only two pins in the plug, so just insert one end of the resistor into each lead of the plug. Wrap up the end with electrical tape to make sure your resistor doesn't fall out. Your ECU will be happy and your EGT probe will never burn up.