The Water Pump has a serious function in your Toyota Avensis as it continuously pumps water mixed with coolant throughout your vehicle's engine so it doesn't overheat. Water pumps can leak and the bearing can become faulty resulting in serious damage to your engine. Water pumps are driven by your cars timing belt or fan belt. If your pump is faulty you should replace it immediately and also replace any belt that drives it to avoid further damage and additional cost. If the Water Pump in your [make Avensis is driven by the Timing Belt, then you must replace the Timing Belt too, as they cannot be refitted.