carver cascade 2?

I have an Elddis Vogue 1V which has the Carver Cascade 2 hot water/heater system. When I switch on I get the green light, then after a few seconds I get a pop/backfire sound and the red light comes on. If I try with just electric there is no pop sound but I still get the red light. There is gas in the bottle as I'm getting gas to the hob, and tank is full of water. All electrical connections look ok. Any ideas?