SLOWING PROBLEM PEUGEOT 306

If you've already ruled out the IAC valve, then check that all cylinders have spark and good fuel injection, see if the idle improves. The last thing you should do, and it's not highly recommended, is to adjust the idle screw to raise it a little more; it's not recommended, but it can be done.

I found out what was happening. My car had an LPG system installed, and the diffusers were loose on the intake manifold, so air was getting in and leaning out the fuel mixture.
Thank you so much for everything.

I finally figured out the problem with my Peugeot's idle; it was the easiest thing to do. Just resetting the IAC valve makes everything work normally again... but to do this, you have to disconnect the battery and leave it disconnected for a few minutes. Disconnect it from the negative terminal; I always did it from the positive terminal, and that's why the idle wasn't working properly. So, you can't reset it from the positive terminal. Thank goodness I found the problem... I've had 20 cars, and none of them gave me as much trouble as this one.

CHECK THE THROTTLE BODY (THE FLAP) IF IT'S DIRTY IT WILL ALSO CAUSE IDLE PROBLEMS