How do I get my fluorescent 4-tube under-cabinet lights to come on reliably?

When they have been on a while and just been turned off they will reliably come on again immediately with no flicker. When they have been off for a while, they will not come on reliably. I plug and unplug repeatedly and will sometimes get a flash of light, sometimes not. Finally one of the times I replug, the lights will all suddenly come on with no flicker. Once on they stay on fine.

This has gradually gotten worse so that now often I cannot get them to come on at all. These were working fine and have not been disturbed except that the power went off when they were on. They never come on when I leave them plugged in -- only when just plugged in. Other things work fine in that outlet and the same problem occurs if I plug the lights into other outlets. I have replaced the plug, cutting off the last bit of wire coming into the plug. These are at room temperature (70-71 degrees.) This is an expensive unit with wood ends that match the cabinets so I would prefer not to replace the unit.

This is urgent as it is the sole lighting for indoor bonsai trees which are starting to lose leaves from lack of this light. (I've tried moving them elsewhere temporarily but they are valuable and very sensitive and need their watering recalibrated for the temporary location's conditions and meanwhile they are declining. I want to put them back in their permanent location as soon as I can.)

  • 1
    If you're using the lights for bonsai, does that mean they have a programmable controller? You can get weird voltage spikes during power outages that can damage electronics.
    – Niall C.
    Commented Nov 8, 2012 at 15:02
  • No, nothing fancy. When we installed the fixture we just hooked it up to an extension cord with one end cut off and plug it in in the morning. But it had been working fine!Thanks.
    – M_SD
    Commented Nov 8, 2012 at 21:56

3 Answers 3


It sounds like you may need to replace the ballast. They do age and eventually do need to be replaced, though not nearly as frequently as the tubes themselves.

fluorescent ballast

  • I thought if it was the ballast it would flicker?
    – M_SD
    Commented Nov 8, 2012 at 15:02
  • If it gets really bad that may be the case but the first symptom is typically that it has trouble starting up. Commented Nov 8, 2012 at 17:38
  • OK, so even though when it does start it comes on instantly everyone thinks that's the ballast?
    – M_SD
    Commented Nov 8, 2012 at 21:54
  • If you decide on a replacement fixture beware of the very lowend models. My recent experience has been that cost directly relates to reliability. You don't need the most expensive, just something that will last.
    – mikes
    Commented Nov 8, 2012 at 22:39

Basic principles

fluorescent lights work by exciting mercury vapor, instead of simply heating an element like in an incandescent bulb. In a metal conductor; like that used in an incandescent bulb, the resistance changes very little. This means the bulb will draw the same amount of current for the entire times it's on. In a fluorescent tube, current causes the resistance of the gas to decrease. If this was left uncontrolled, the resistance would eventually drop to 0 Ohms, the current would then be infinite, and something awful would happen. This is where the ballast comes in.


A ballast uses either magnetism or fancy electronics to control this raise in current, preventing the mayhem that would ensue if an infinite amount of current was allowed to flow. How ballasts actually work is a bit in depth, so is outside the scope of this answer. For now, we'll just call it magic. If you'd like to have a better understanding of how a ballast works, you could start by learning about inductors.

Ballast image

Because the current to the tube must be limited, the limited current may not be enough to ionize the mercury vapor and light the lamp. This is why starters are often used.


A starter is basically a time-delayed switch, which bypasses the ballast for just long enough to get the lamp started. A common type of starter is called a glow tube.

Starter image

A glow tube is a normally open switch, which is activated by applying a current. When current is applied, one lead begins to glow, heat, and bend. This lead eventually bends far enough, and makes contact with the second lead. When this happens the first lead cools, and bends back to it's original shape. The inductive kick generated by the leads disconnecting starts the main discharge in the fluorescent tube, and the lamp is lit. This (very exciting) YouTube video shows how a fluorescent starter works.

Glow tube image

In modern fluorescent fixtures both the ballast and starter are made up of electronic components, and are often housed in a single container and may even be part of the fluorescent fixture itself.

Diagnosis and repair

Warning: Electricity is dangerous. Working with it can lead to property damage, injury, and death. Please, never be afraid to contact a qualified Electrician.

If the light is turned on and the ends of the tube glow, the bulb flickers, or the bulb simply never ignites. You'll want to start with the simplest, and cheapest solution first. In this case, that would be to replace the bulb(s). If that does not fix the problem, you'll have to replace the starter, ballast, or the entire fixture. If the starter is a separate unit, try replacing the starter first. If the starter and ballast are a single unit, replace the starter/ballast unit. If the starter and ballast are integrated into the fixture, you'll have to replace the entire fixture.

  • If I have 4 bulbs and they sometimes all light, and sometimes (usually) none light - that wouldn't be an indicator of a bulb problem I assume, right? When they don't light, two of the bulbs have glowing ends - it's as if whatever that mechanism is only exists in two of the bulb mounts... Is that common, or an indicator of anything in particular? Thanks Commented May 17, 2018 at 7:38

Could be a problem with the ground to the light fixture case. I had the same problem once and put in zero degree ballasts and everything but nothing helped Then an older electrician friend said it HAD to be the ground and it turned out that he was exactly right. Whoever installed the light had not hooked up the ground to the fixture case.

  • More story than answer, but there is an answer in there I suppose... @user12139, try to make answers less wordy and get to the point in the future...
    – ShoeMaker
    Commented Mar 19, 2013 at 23:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.