For those not fluent in french :
You need to trade-in a car or a van you own since at least a year and it needs to be from before 2006 or 2011 if it's a diesel. Then 40% cap is 1500€ for everyone and if you are way below minimum wage it's up to 3000€. An additional 1000€ might be available depending where you live (low emission zone which include at the moment 10 of the biggest cities and more to come).