It looks like that sensor can cause ALL of the symptoms you described. One wouldn't normally think so, being just a temp. sensor, but I suppose it's input is used for more than just a simple "how hot is it?"
Info:
Quote:
DTC P0546 - EXHAUST GAS TEMPERATURE SENSOR CIRCUIT HIGH-BANK 1
DTC DETECTING CONDITION:
Immediately at fault recognition
TROUBLE SYMPTOM:
Hard to start
Erroneous idling
Poor driving performance
CAUTION: After repair or replacement of faulty parts, conduct Clear Memory Mode and Inspection Mode.
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The last part under "Caution" means that it won't reset without using an actual OBD scanner, and clearing the self learning of the ECU. Sometimes, with hard codes, you can turn the light off by disconnecting the battery, but the ECU needs an actual reset to start thinking right again.