Getting a P0505 code:Air control system. Replaced IAC and still get the same code. Idle is rough and gas mileage is down to 4 miles to the gallon.

P0505 should be the IAC (Idle Air Control) System Malfunction

Hope this helps

Error Code: P0505
Fault Location:
Idle Speed Control (ISC) System - Malfunction
Possible Cause:
-Wiring harness.
-Idle speed control (ISC) actuator/Idle air control valve (IAC) valve.
-Throttle valve tight/sticking.
-Engine control module (ECM).

Error Code: P0130
Fault:
Oxygen Sensor (O2S) 1, Bank 1 - Circuit Malfunction
Possible Cause:
Malfunction Indicator Lamp (MIL) Triggered on first occurrence.
-Wiring harness.
-Oxygen sensor (O2S).
-Engine control module (ECM).

Error Code: P0505
Fault:
Idle Speed Control (ISC) System - Malfunction
Possible Cause:
-Wiring harness.
-Idle speed control (ISC) actuator/Idle air control valve (IAC) valve.
-Throttle motor.
-Throttle valve tight/sticking.
-Engine control module (ECM).

Error Code: P0100
Fault Location:
-Mass Air Flow (MAF) Sensor/Volume Air Flow (VAF) Sensor - Circuit --Malfunction
Possible Cause:
-Wiring harness.
-Mass air flow (MAF) sensor.
-Volume air flow (VAF) sensor.
-Engine control module (ECM).

Error Code: P1491
Fault:
Exhaust Gas Recirculation (EGR) Valve Insufficient Lift
Possible Cause:
-Wiring harness.
-EGR Valve.
-ECM

Error Code:P1607
Fault:
-Engine Control Module (ECM)/Powertrain Control Module (PCM)

Error Code: P0966
Location:
-Pressure Control (PC) Solenoid B - Circuit Low
Possible Cause:
-Wiring short to earth.
-Pressure control (PC) solenoid.
-Engine control module (ECM)/Powertrain control module -(PCM)/Transmission control module (TCM) .

This is common issue with Infiniti I30 1999-2001.

When the coil packs fail they burn up components/parts on the engine computer ECU. If the problem gets worse, you will need to replace the ECU or have it rebuilt.

Is your check engine light on?
You might also get code P0505
Technical bulletin also recommends to replace
The Idle Air Control Valve MUST be replaced prior to re-installation of the ECM, or immediate damage to the computer will occur again.

http://www.myairbags.com/product/ecu-infiniti-i30-2000-2001
Common Failures:

• No Start, Hard Start, Engine Stalls
• Rough Idle, High Idle, or Idle Up and Down
• Misfire to one or more cylinders
• No signal to the ignition coils
• Engine Trouble Codes For Idle Control Valve (IACV)
• No Communication With Computer
• Replacing IAC Does Not Fix The Problem

P0505: Idle Air Control Valve System fault, Code P0505
-Vacuum leak
-Dirty throttle body
-IAC motor defective
-Poor electrical connection to the IAC valve, hope this helps and good-day !

Code P0505 is Idle Air ControlSystem. P0300 is Random Misfire. P1151 is Lack of HO2S21 Switches- Sensor Indicating Lean. Possible cause is Idle Air Control solenoid.

I had a similar problem on my Aurora 4.0. These had a common problem with the air intake plenum gasket leaking. The car, in this condition, couldn't measure the air flow properly and the result was a poor air/fuel mixture. This is easily seen with an ODB-II scanner looking at the fuel trims and O2 sensors.
The SES light means you are getting CODES. These are certainly going to lead you in the right direction.
The hiccup sounds to me like maybe a crankshaft position sensor - you should get this fixed ASAP if it is, as you can trash the engine if you don't. It will throw the appropriate code, so start with getting the codes scanned.

per this address "http://www.obd-codes.com/trouble_codes/" p0505 is a "IAC (Idle Air Control) System Malfunction" it gives some good suggestions to look at to find a solution for your problem.

YOU HAVE INTAKE VACUUM LEAK AT THROTTLE BODY OR PCM HAS FAILED.NOT CONTROLLING IDLE SPEED CONTROL CIRCUIT.

P0505 is correct, but replacing the IAC won't help if the PCM cannot control and/or monitor the operation

you need to measure voltage at the IAC and/or the signal return to the PCM