Question one: is there power at the driver output?
Measure at the driver's low voltage terminals with the strip connected, then again with it disconnected. Three outcomes matter.
Nothing on either test sends you to the dark run branch in why an LED strip will not turn on. Correct voltage with the strip off but a collapse when connected means the load is pulling the supply down, from an overloaded driver or a short. A reading that starts correct then falls away after minutes, often with the driver warm, is the thermal branch in an LED driver that got hot and shut down.
Confirm the obvious while you are there. The strip's rated voltage and the driver's output have to match, 24V with 24V or 12V with 12V. Multiple runs on one driver sit in parallel branches, never chained. Drivers and strips both generate heat, and ventilation and clearance are published per unit (verify the driver's rating, clearances and the current headroom guidance before you size anything).
Question two: does it light, and is it all of it?
A dark run on a healthy supply points at the joint between them: the connector, the polarity or the first foot of tape.
Light that stops partway is the half lit branch, and the boundary is the evidence: only half the LED strip lights up covers what to read there. A run that lights end to end but fades along its length is voltage drop rather than a fault, and belongs in an LED strip dimmer at one end. A sharp stop is a break; a gradual fall is a feed problem.
Question three: is it constant, intermittent or triggered?
Constant faults are the friendly ones. They sit still while you test them, and the first two questions usually catch them.
Intermittent faults need a pattern, not a poke. Note what the run does and what changed just before, over a couple of days if necessary. Flicker that follows movement is mechanical, usually a joint. Weather points to thermal. The clock points to control.
Triggered faults have an author. If the run switches on with nobody asking, an LED strip that turns on by itself lists the five places that command can come from.
Question four: does the control change anything?
Bypass every control device and run the strip straight from a suitable driver. What happens next splits the tree.
If the symptom disappears without the control, the fault is in the control chain: the dimmer, the controller, the pairing or the combination. Audible noise under dimming is its own branch in why an LED strip buzzes when dimmed. If it survives the bypass, the control is innocent and you are back in the power and wiring half of the chart.
Rule one combination out early. A smart low voltage controller fed through a phase cut wall dimmer produces symptoms nobody can trace unless both manuals sanction that arrangement, so treat the two as separate control methods. Low end behavior depends on a verified chain of strip, driver, control device and protocol.
Question five: what does the tape itself say?
If all four answers point back at the strip, compare the run against the product it is. Watts per foot, maximum run length, cut spacing and connector fit are published per product and vary between them. Browse by rating rather than appearance in the LED power supplies collection, and where a replacement length is needed the LEDClub 24V COB 300 Series suits medium output interior work (verify its published watts per foot, maximum run and cut spacing).
