I bought a 2.4 GDI Galant (year 2000, manual) about 4 months ago with 120 000 km. I bought it in Germany and wanted to register in another country and the problems started to unfold only during registration procedure as I had to take it to inspection where it turned out the emission rate (all components) is too high.
I took it to a Mitsu dealership. They said the katalysator and O2 sensors are fine. There is also some problem with rough idling, so I think the two problems are connected. I have been reading through loads of posts on different forums and noticed that many people had similar problems with idling. I have let the Throttle body and AIC cleaned, checked valves. In the dealership they said electronics are fine too. So now I ran out of ideas. The local dealership is not much help either. They said it is possible that the whole AIC motor has to be replaced but it costs something like $ 2000. Does anyone have any idea what can be the problem? Does the AIC really need to be replaced?
Any suggestion is much appreciated, because this problem is really keeping me awake at night!

New developments:
Yesterday I went back to the dealership to ask what exactly was wrong with my AIC, that the whole thing needs to be replaced. The guys there are apparently quite pissed by the fact that Im appearing so frequently, so now they have checked through everything (dont know why they could not do that for the first time...) and it turned out the valves have some problem. The funny thing is, that last week I was specfically asking him about the valves and he said he doesnt hear this specific nosie, so valves are ok. Now he said the opposite.
Anyways, so he said they need to be replaced. Then I asked the guy, if its possible first only to dismantle the engine and chack whether the valves are indeed broken or just need to be cleaned. He said no, they need to be replaced.

My question is: Can it be really judged only by the sound the engine makes, whether the valves are broken or just too much dirt built up?

Rough idling doesn't mean it's necessarily the valves. What usually fixes the probel in cleaning out the throttle body and the AIC.

Have you done a tune up? Perhaps the spark plug/wire need replacement because they also effect emissions and engine performance.

IAC and throttle body have been cleaned
Spark plugs have been checked as well, but apparently they are fine.
Valves have been replaced.

But the problem still exists.

The guys at mitsubishi said there is some problem with the compression in the first cylinder. They think it might be a faultry injector. Honestly I dont trust those guys anymore. Apparently there is a mechanic here who is able to localise the source of high emission with a computer. Maybe that will bring me closer to the solution, however any suggestion is still highly appreciated!!

Try using higher octane fuel even one of those fuel injector cleaners or anti-H2O additives might help. Maybe you've got some water in your tank. ... Just thoughtof it maybe PCV valve there often overlooked.