P0506 CEL after replacing catalytic converter and O2 sensors

[MrTony]

I recently replaced the passenger side catalytic converter (old one had a hole) and the upstream and downstream O2 sensors. Since then, the engine repeatedly surges to 2100 RPMs then drops to 1500 RPMs when the car idles (thanks for the responses to my earlier post). I've driven it a few times but the problem persists. I disconnected the battery for over an hour to reset the computer. The CEL went out temporarily but soon returned. I'm receiving a P0506 code (P0506 Idle Air Control System RPM Lower Than Expected). Just wondering if anyone knows why this would be the case. I'm also wondering if the P0506 code is telling an accurate story.

Thanks in advance.