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We have a couple of old fluorescent fixtures in our basement, each holding four 4-ft long T12 bulbs. After several weeks of slow starts, the lights finally stopped coming on in one of them so while we eventually will replace the fixtures with LED ones, I went ahead in the interim and purchased four new bulbs. I put two of the new bulbs in the two adjacent slots on one side of the fixture, thinking that it needed both bulbs to be in at the same time. When I turned the fixture on, one of the bulbs looked like it was trying to light up, the other did nothing. So I concluded it was the fixture and removed the new bulbs.

Now I am wondering if I was too hasty in that decision: Is it possible that all four bulbs have to be in place for them to light up or maybe the two inner ones or the two outer ones have to be in place, instead of the two on one side as I tried. Is there some standard in what these fixtures expect?

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    It depends on the ballast: some require two bulbs, some need all four, to light correctly. But since the fixture is already disassembled, and a no-mercury, efficient, color-correct LED fluorescent tube replacement can be found for <$US10 per tube, why not do it now?
    – DrMoishe Pippik
    Commented Dec 21, 2020 at 2:00
  • Fixture wasn't disassembled - I just opened the diffuser.
    – Not_Einstein
    Commented Dec 21, 2020 at 16:06

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These fixtures are usually wired so that the (2) outside bulbs and the (2) inside bulbs are wired together. Yes both bulbs need to be inserted in the sockets to light-up. Also the 4 foot bulbs with (2) terminal pins need to be turned so that they lock in place and contact the terminals carrying the voltage. Usually a 1/4 turn or until resistance to turning is felt.

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  • Yes ! When I put the two inner bulbs in they lit up. Of course when I then also put the two outer ones in the two inner ones didn't light but it apparently was just that one of them needed to be reseated a few times. Now all four light up.
    – Not_Einstein
    Commented Dec 21, 2020 at 16:10
  • Glad to hear it. Additionally these (4) foot bi-pin bulbs are notorious for not liking a cold environment nor being switched on/off alot (both shorten their lifespan). Best remedy is to purchase brand name bulbs. (Or change to LED as planned!).
    – ojait
    Commented Dec 21, 2020 at 17:05

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