LED Not Lighting Up: 7 Causes and How to Fix Each One

Quick answer: nine times out of ten a dead LED is reversed polarity, a missing/open series resistor, or a broken solder joint. Check those three before anything else.

The 7 Causes, in Order of Probability

• 1. Reversed polarity — flip the part or check the cathode marking. Verify with a multimeter in diode mode.
• 2. Open series resistor / no current path — measure resistor continuity; a missing current limit often kills the LED instantly, so replace both.
• 3. Cracked solder joint — reflow with flux; look for tombstoning or pad lift on SMD parts.
• 4. Damaged die (ESD or over-current) — diode mode shows OL in both directions or a short. Replace the LED and review handling.
• 5. Driver output fault — measure the voltage at the LED pads with power on; zero volts means the fault is upstream.
• 6. Vf budget too tight — e.g. a white LED (3.2 V) on a 3.3 V rail with a dropping resistor that is too large. Recalculate against the datasheet Vf max.
• 7. Heat damage from soldering — failures right after assembly usually trace to excessive reflow peak or a hot iron held too long.

30-Second Diagnostic Sequence

Visual inspection → diode-mode test off-board → voltage across LED in-circuit → voltage at driver output. This isolates LED vs circuit in under a minute.

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