Photocell took a crap. Other times I'll get a call where they say their exterior lights won't turn off. Go down, find the photocell, disconnect it, lights turn off. Replace the photocell with a new one and simulate night/day and the lights turn on and off accordingly.
Since a photocell simply detects the ambient light intensity to turn the lights on and off, it may take that artificial light source as a stimulus and get turn the light off, considering that there is already enough outdoor light.
Ensure that the photocell is functioning and is reacting properly with the light level changes by covering the photocell, blocking any light going to it. When covered, the reading should drop down to “0”. If you shine a light on the photocell, the reading should go up to around “1020.” (Please refer to the “Checking Photocell Reading” section)
For example, a loose wiring, or wrong wiring can result in a totally nonfunctional photocell. Similarly, debris accumulation over the time may cause a photocell to stop working. However, where these might be the common reasons of malfunctioning photocells, make sure that the problem doesn’t lie with your light fixture.
Overly Sensitive Photocell Sensor Sometimes, a photocell may cause a light fixture to flicker, or go on and off at unusual times. If a photocell is making the lights flicker too often, one possible reason behind it could be a too sensitive photocell. It makes the light go off and on with slightest changes in outdoor light intensity.
The photocell will not give the correct reading if the leads are reversed, wires are shorted, and/or if a ground loop is present. Move the photocell and plug it directly into the photocell input (PCC1, PCC3-de, Blue Box LT or iDIM/iDH) eliminating the field wires, and follow step #1.