HOW TO RE-LEARN THE IDLE (Reset the ECU) Honda City (2009 to 2017):
First clean the Air intake block and throttle body and install a new Air Filter, after this you feel the high RPM now you can follow the below instructions.
Step-1: Remove the battery negative terminal for more than 10 seconds. Preferably wait for about 10 minutes before re-tightening the battery negative terminal.
Step-2: Start your car and warm up till you can hear radiator fan turning on it might take 2 to 10 minutes. (Do not use any electrical accessory (AC, Power Windows, Radio).
Step-3: Turn off the car remove the key and restart it again. Turn on the car again and stable the RPM at exactly 2000 for 10 seconds and turn off the car and remove the key.
Step-4: Finally restart the engine to verify the idle speed at 800 RPM, Now ECU/Engine idle is reset to its default settings.