Hi recently my pre paid gas ran out. I checked and I had 15 credits. I know my mums boiler had the same issue where it shuts off if there are only 15 credits left. She tells me an engineer comes and does something that takes 2 seconds and its working but he never told her why. Its a Vaillant boiler I tried topping up another £10 and put the card in to release gas and gas meter shows "o" which I think means on. No gas appliances in house to check gas supply. Anyone any ideas?