Motion Sensor Light Stays On After Storm Decision Tree

Why this matters

A motion-sensor exterior light that stays on after a storm and refuses to time out is responding to a hardware change, not a misconfiguration. Three failure modes account for nearly every storm-correlated case: water intrusion into the PIR sensor head shorting the trigger logic, a surge-damaged photocell that has lost its high-impedance OFF state, or a microwave-PIR combo head whose RF section is now unbounded. The light cycles correctly on dry days because the failed component happens to look like a held-trigger only when damp. Replace the wrong part and the light stays on through the next storm too.

Symptom presentation

Confirm the storm correlation. The classic case: light stayed on all night after a thunderstorm, came back to normal the next morning when the sun and wind dried the housing. Then on the next storm two weeks later, it stayed on again. If the light only stays on after consecutive humid days without rain, the symptom is in the same family but the workup includes housing condensation as a possible source.

Note the fixture style. Outdoor PIR floodlights from Heath/Zenith, RAB, Halo, and Lithonia all use a similar topology: PIR sensor on the head, photocell on top, and a relay that latches ON when motion + low light coincide. The combo-detection units (PIR + microwave) add a second source of false-on under wet conditions because microwave detection is sensitive to moving water droplets in the beam.

Quick checks

Power off at the breaker. Open the head and inspect the sensor module. Three red flags: water staining inside the lens dome, white powdery oxidation on the sensor PCB pins, or cracked silicone at the head-to-arm seal. Any of these confirms moisture has reached the electronics and the head needs replacement; PIR modules are not field-rebuildable.

Inspect the photocell. Most exterior fixtures use a CdS photocell shunted across the sensor input. A surge-damaged CdS goes from megohm OFF to kilohm OFF and the relay never resets. Measure the photocell in darkness; a healthy CdS reads above 1 megohm in dark and drops to under 10 kilohm in sunlight. A photocell stuck at 5 to 20 kilohm in darkness has failed.

Check the gasket on the conduit entry at the back of the housing. A failed gasket lets driven rain into the box behind the sensor and shorts the line side to the trigger logic. This is the most common storm-correlated cause on Heath/Zenith and Halo fixtures more than 5 years old.

Isolation tree

Replace the photocell if it tested bad. Power back on at dusk and watch the light cycle. If it now turns off at dawn correctly but still false-triggers in motion, the PIR module is the next candidate. If photocell tested good, jump to the PIR.

Test the PIR by covering the lens with opaque tape and motion-triggering manually. With the lens fully blocked, the relay should not latch on motion. If it latches anyway, the PIR sense path is compromised; replace the sensor module or the whole fixture.

For microwave-PIR combo units, the microwave element fires through clear plastic and is not blocked by the lens cover. Tape will not isolate the microwave path. To test, disconnect the microwave module's trigger lead at the relay board. If the false-trigger stops, the microwave element is the cause; if it continues, the PIR is the cause. Some integrated boards do not allow separation; in that case replace the whole head.

Confirming diagnosis

Force the storm. Spray water with a hose on the suspected gasket entry for 5 minutes while the fixture is energized at dusk. If the light latches on under the spray, the gasket and conduit seal is the cause. If the light cycles correctly under the spray but failed during the actual storm, suspect surge damage to the PIR board, not water; install a Type 3 SPD at the fixture branch and replace the PIR module.

For surge-damaged units, the failure mode is binary: the unit either holds OFF or it does not. A unit that intermittently false-triggers based on weather has water intrusion, not surge damage. Surge damage shows up as a permanent symptom that does not improve when the housing dries.

Remediation

Replace failed sensor head or whole fixture. Re-gasket the conduit entry with a butyl sealant or a UV-rated silicone (not standard silicone, which degrades in 18 months of UV). Drill a 1/8 inch weep hole at the lowest point of the back-box if the fixture does not have one; trapped water above the gasket level will keep finding ways in.

Install a Type 3 surge protective device (SPD) at the branch panel feeding the fixture if the home has lost multiple sensor heads to lightning. NEC 2023 Article 230.67 mandates Type 1 or Type 2 SPD at the service for new dwellings; Type 3 at the branch is the cascade that protects the actual electronics.

For premium fixtures (RAB Stealth, Lithonia OFL2, Halo HU), follow OEM installation instructions for gasket replacement parts; buy-vs-replace at MSRP for a 10-year-old fixture is rarely worth a rebuild and a new fixture comes with a fresh warranty.

References

• NEC 2023 Article 230.67 (Type 1 or 2 SPD at service entrance), 408.36 (SPDs in panelboards), 410.10(A) (outdoor luminaires wet location listing)
• UL 1598 (Luminaires; wet-location listing requirements)
• UL 1449 (Surge Protective Devices, Type 1, 2, and 3 ratings)
• ANSI/IEEE C62.41.2-2002 (Recommended Practice on Characterization of Surges in Low-Voltage AC Power Circuits)
• Lithonia OFL Series Installation and Maintenance Guide (gasket replacement and weep-hole guidance)