Why Does Half My Christmas Tree Go Dark? (And How to Fix It)

Half a Christmas tree going dark is one of the most common holiday frustrations — and it’s usually caused by one failed bulb in a series circuit, not a wiring problem. Here’s why it happens and how to find and fix the bad bulb quickly before the season is over.

Why half the strand goes out from one bad bulb

Most traditional mini-light strands wire bulbs in series — meaning current flows through every bulb in a row before returning to the plug. If one bulb’s filament burns out, the circuit breaks and the entire section goes dark. Modern strands use “shunted” bulbs: each socket has a shunt — a small piece of metal or resistive wire — that redirects current around a failed bulb, keeping the rest lit. When the shunt itself fails over time, the whole section goes dark like a classic series circuit failure.

Finding the bad bulb

The fastest tool for this job is a light strand tester — a small probe tool ($10–$15 at hardware stores) that you run along the dark section. It detects the break point and usually identifies the failed bulb within seconds. Without a tester, the classic approach is substitution: start in the middle of the dark section, replace a bulb with a known-good spare, and check if the section lights. If not, move to a different bulb. This is tedious but reliable.

A multimeter on continuity mode can test individual bulbs out of their sockets: a good bulb shows continuity (a beep or reading), a failed bulb shows open circuit.

What to do if no bad bulb is found

If you’ve tested every bulb and they all show continuity, the problem may be a corroded or loose socket rather than the bulb itself. Clean the socket contacts with a cotton swab and electrical contact cleaner, reseat each bulb firmly, and retest. A damaged wire inside the strand — from being stored coiled tightly or pinched under furniture — can also break the circuit; inspect the wire in the dark section for any visible kinks or damage.

Expert tip

Once a Christmas light strand starts having sections fail, it’s usually more time-effective to replace the strand than to keep troubleshooting. Modern LED Christmas light strands use parallel circuits or constant-current LED drivers so a failed LED doesn’t take out a whole section. They also use 80–90% less energy than incandescent mini-lights and last significantly longer. A 100-light incandescent strand draws about 40W. A 100-light LED strand draws about 4–5W. If your strands are more than 5–7 years old, switching to LED is worth it for the reliability and energy savings alone.

Contact us

Planning an outdoor holiday lighting setup that works all season? Call us at 416-838-9006 or visit our contact page — we install permanent and seasonal outdoor lighting across the GTA.

Amir Azimipour

Electrician Since 2008 Journeyman Electrician Designated Master Electrician at EZSMART Corp