The other day I did an oil change on my 406. The nice people who did my clutch replaced some of the quick release fasteners for the undertray with nuts and bolts. The end result was that the job took longer than I was expecting, so when I needed to pick my gilfriend up from work, I went without putting the undertrays back.
The air con went from being pretty good to absolutely freezing. Must be getting a lot more airflow through the radiator and condensor.