When half a zone goes dark overnight, the problem is almost always at one point on the run, not spread across the system. The lights between that point and the transformer still work. Everything beyond it does not. That pattern points to four likely failures, and the diagnostic walk takes about 20 minutes.
Confirm the Pattern
Before diagnosing, confirm what you are actually seeing. Walk the affected zone with the lights on, ideally at dusk. Note which fixtures are working and which are not. If the failure is clean – everything past a certain point is dark – you have a break in the run. If the failures are scattered, you have a different problem (typically bulb failures or voltage drop).
A clean break pattern is good news. It means one failure point fixes the whole zone.
Likely Cause 1: Severed or Damaged Wire
The most common cause of a sudden, clean failure. Something cut the wire. The usual suspects:
- Recent landscaping work (edging, planting, irrigation install, mulch delivery)
- Aeration or core service in the lawn
- Rodent or animal damage, especially squirrels or rabbits on exposed runs
- Frost heave that pulled a buried splice apart
- Roots that grew around a connector and pulled it loose
Walk the line between the last working fixture and the first dark fixture. Look for fresh disturbed soil, mulch that has been moved, or any sign of recent groundwork. Probe gently with a screwdriver where you suspect the break. A working wire tracer cuts this search from an hour to ten minutes.
Once found, cut out the damaged section, strip fresh conductor, and splice in new wire with two waterproof, direct-burial-rated connectors. Bury the splice with slight slack so it does not pull tight in cold weather.
Likely Cause 2: Failed Splice
If the last working fixture and the first dark fixture are close together, the splice between them is the suspect. Splices fail for predictable reasons:
- The original connector was not rated for burial and failed when water got in
- The connector worked loose due to thermal cycling
- Corrosion built up over years until contact intermittently disconnected, then disconnected permanently
Dig down to the suspect splice. If it looks green, chalky or the wires pull out with light tension, that is your failure point. Replace with a gel-filled or silicone-filled waterproof connector.
Likely Cause 3: Tripped GFCI or Failed Transformer Tap
Less common when only half the zone is out, but worth checking. If the transformer feeds multiple zones from different taps and only one tap appears dead, that tap or its internal fuse may have failed.
Open the transformer. Check for a tripped breaker, a blown fuse on the affected tap, or a tap terminal that is loose. If your transformer has indicator lights for individual taps, check the labels.
If the transformer itself is plugged into a GFCI outlet, check that GFCI. A tripped GFCI usually kills the entire system, not half, but a partial system failure can sometimes correlate with intermittent GFCI behavior that requires investigation.
Likely Cause 4: Water Damage at a Buried Splice
This is the same failure mode as cause 2 but with a different timeline. A splice that has been working fine for years can finally fail after a heavy rain, or a freeze-thaw cycle drives water into a marginal connector.
If the failure happened after weather and you cannot find a fresh dig site, look for the lowest point along the run. Water collects there. Buried splices in low spots fail first.
The Diagnostic Walk
Work the failure in this order. Each step is faster than the one before.
- Identify the boundary fixture (last working) and the first dark fixture
- Inspect the ground between them for recent disturbance
- Check the transformer and any inline fuses or breakers
- Probe for a buried splice between the boundary fixtures and inspect it
- If still not found, use a wire tracer or call in a pro with one
Repair Best Practices
- When you find the failure and repair it, take five extra minutes:
- Replace the bad connector and any other connector within a few feet that is the same age
- Add a small drip loop in the wire so future thermal expansion does not pull the splice tight
- Note the location for future reference (a simple sketch saves the next person a search)
When to Call a Pro
If the run was professionally installed and the failure point is not at an obvious splice, a wire tracer locates buried breaks faster than digging. Most outdoor lighting professionals charge a flat fee for a diagnostic visit that pays for itself versus a half-day of speculative digging.
