I just got this code on my 03 yesterday too (bought car 2 days ago).... I was told by the previous owner that it happened after putting 87 instead of 93 in a few months ago. However, I understand that 87 is generally accepted in this vehicle with no issues.
Could this be the problem, or am I in need of a more permanent solution (check leaks, sensors, etc)?
The fault detection is either O2 or the CAT. Replacing method would not solve, as to confirm the fault which has enabled this condition, certain tests need to be done to reveal during Engine warm up and the engine RPM below 4000 RPM pattern post warm up of the respective waveform from the O2 sensors. this would reveal whether the CAT (three way catalytic converter) requires replacement or the O2 itself. You need to get the Freeze frame data from the OBD scan tool of the engine condition whether the air fuel ration was lean or rich and other data when the malfunction occurred. Probably you could start with a good electrician to see which one is deteriorated. And then replace.