1999 Civic EX suddenly started acting strangely while throttle is just off idle, during downhill driving. Seemed like there wasn't the normal gradual ramp of throttle response, it was either on suddenly or off all at once. Then when I got home, it would idle erratic like it was "hunting" as describing diesel engines or steam turbines with governor problems. The code is P1509, idle speed control auxilliary inputs. I made sure the coolant was full, vacuum is OK, fuel filter is new, valves adjusted. Took the throttle body apart and it is pretty clean. Took the IAC Valve off and it was clean too. Unplugging the IAC valve doesn't change the idle.

What components feed the IAC circuit? Could it be one of the O2 Sensors, or the Cat? The past couple months the check engine light has been on, code indicated the Cat efficiency was off. Didn't replace it yet. Should I replace the IAC Valve?

Thanks for any advice.