Renault Clio engine fading occassionally when trying to accelerate

Just acquired the car for my son whose just started driving. Sometimes the engine will not deliver the power when the accelerator is depressed and it's as if the oomph has been taken out of it. After stopping it usually starts aain in a short while. This seems to happen mostly in the damp so thinking it might be water getting into wiring or affecting the ECU. Where are the weak spots for water ingress to reak its havoc?

Also I have the toxic fumes light on all the time. Thinking it might be plugs, lambda sesnor, TDC sensor. Am I in right general area?

Lastly, after depressing my warning flashers button (after a fade) I find it wont turn off again unless the button is held down. Is it easy to just change the button (breakers yard)or do I need the whole panel?

Thanks for any knid contributions.