LED not lighting? Trace the circuit before replacing it

Start with a complete low-voltage path

A dark LED does not automatically mean a dead LED. Sketch the intended loop: source positive → current-limiting resistor → LED anode → LED cathode → source negative. The resistor can instead sit after the LED, provided it is in series. Mark each distinct node on the sketch and compare it with the real connections. This is fault-finding, not resistor selection: do not remove or bypass the resistor to see whether the LED gets brighter.

Switch off and disconnect the source before moving leads or checking continuity. Use an appropriate isolated low-voltage supply; do not troubleshoot this example on mains wiring or an unprotected high-energy battery.

Check polarity without trusting one visual clue

In the intended forward direction, current enters the anode and leaves the cathode. Many through-hole LEDs have a longer anode lead and a flat near the cathode, but trimmed leads, unusual packages, and damaged markings make those clues unreliable. Check the part's data sheet or a correctly configured meter's diode-test function with the circuit unpowered and the part isolated if other branches could affect the result. Do not apply a large reverse voltage to test whether it works.

Follow the hidden breadboard metal

On a common solderless board, a numbered row has one connected group of five holes on each side of the center trench. The two groups across the trench are separate. Put the resistor's ends on different nodes; both ends in one five-hole strip bypass that resistor. An LED lead placed one row away from its intended jumper is often completely disconnected, even if the layout looks almost right. Boards vary, so verify uncertain strips with continuity mode while power is removed.

Look for a split rail or missing return

A red or blue rail marking does not prove the rail is continuous end to end. Some boards split a side rail in the middle, and rails on opposite sides are usually not joined unless you add a jumper. With the supply disconnected, test continuity across the intended rail segment and trace the LED cathode all the way back to source negative. A ground-colored wire or GND label cannot substitute for that physical return. Also inspect spring contacts and loose jumpers before assuming the component has failed.

1. Disconnect power and compare every lead with your node sketch.
2. Check the center trench, five-hole groups, and any rail split with continuity mode.
3. Restore only the intended series path, then reconnect the safe low-voltage source.

Measure voltage across each part, not just at the source

After checking wiring, put a correctly rated meter in DC voltage mode and measure in parallel across the resistor, then across the LED while the circuit is powered. For an illustrative regulated 5 V source, 330 Ω series resistor, and an LED operating near 2 V, expect roughly 3 V across the resistor and 2 V across the LED, or about 9 mA. The LED voltage depends on the actual part and temperature. A near-zero resistor voltage may mean an open branch or two resistor ends on one node; a full supply reading across one part may indicate a break elsewhere. Interpret both readings against the traced topology before changing parts.