help cat light on car car wont start!!!pollution message..peugot206?

HELP my husband has just went to start the car and it wont the cat light comes up and a pollution message , it will start but then cuts off. a few weeks ago the engine kept getting hot especially if sat in traffic etc but a mechanic looked and said as was running ok not to worry as didnt happen ofter (dont know if this is related?) anyway when it got hot we looked at the fluid bowl thing at the back of the engine and the fluid kept going to the top of the max line but when cold it would go back down to normal level! any help greatly appreciated!!!