Although the tubes in your fly zapper may appear to be working and giving out plenty of light after use of around 6000 hours run time the amount of useful insect attracting UV light coming from your old lamps drops to a level where it will no longer be attracting insects to your bug killer These Replacement UV blacklight bulbs for bug killer machines fit most  16 Watt fly zappers that use 8 Watt 300mm T5 bulbs such as Easyzap CE983, P154, P313, Y724, Y727, CB827, CB828, CB829, CB830, CB897, CB898.

Specifications:

• Wattage: 8 Watts
• Bulb length: 30cm / 12 inches
• Weight: 60g
• Average life: 6000 hours
• Cap type:  G5
• Quantity: 2 tubes
• Type: T5

It is also advisable to replace the starters in your insect killer when replacing the bulbs as your old starters may not be able to provide enough power to reliably light your new lamps. See this other listings for compatible starters www.ebay.co.uk/itm/293129730922