I know this post is pretty old, and I'm surprised you didn't get any responses. I think you may have received some bad information. The P0154 code is
02 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)
The bad cat code is usually
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
Since the code you references is for Bank 2, I believe you were told the wrong code. 4 cylinder engines only have 1 bank, not 2. The second bank is usually the right side of a V engine set up (either V6, V8, V10, etc). Go to autozone or checker or something and have the code read for free, then post the real trouble code if you still have the problem.