2000 Renault Scenic Engine coolant disappearing

Any white smoke at the tail pipe and overheating problem? If you're not leaking then you're burning it in the combustion chamber.

Get it block or pressure tested.

For a start replace the thermostat and also might want to bleed the cooling system.

Always bleed air from cooling system after replacing coolant. Set heater for maximum heat. Remove radiator cap. Loosen drain plug and remove drain bolt (if equipped) from engine block. Drain coolant reservoir. Fill coolant reservoir to MAX mark with 50/50 water-coolant mixture. Loosen bleed bolt and fill radiator up to base of filler neck. Close bleed bolt when coolant flows out without bubbles. Tighten bleed bolt. With radiator cap removed, start and operate engine to normal operating temperature. Add coolant if necessary and check for leaks.