When heater try's to ignite the selenoid keeps clicking in and out quickly not keeping gas valve not to remain open,does this indicate faulty gas valve?

Statewaterheaters.com is a very handy web site where you can look up your instruction manual and find trouble shooting tips. Not sure what model you have, but a dirty or bad flame sensor causes the symptom you describe. A gentle cleaning with some steel wool could do the trick. But get ahold of the manual. It's a big help.