There is no point guessing as it can cost a fortune as you say,you don`t say what sort of code reader was used but it may need to go to a main dealer for the correct computer to see what fault codes there are,have seen different code readers pick up different codes,as to your fault it is very difficult to determine,I would check all pipes/hoses for chaffing and drawing air,not sure on a Renault but filter heads are a favourite for this,other than that it really needs those codes accessed,good luck.
cdc
August 2013
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