It's likely, but not certain, that the photocell needs replacing if your security light won't turn off during the daytime. Here's why:

Possible Reasons:

* Faulty photocell: This is the most common cause. The photocell is responsible for detecting light levels and switching the light on or off. If it's malfunctioning, it might not be sensing the daylight properly.

* Dirty photocell: Dust, dirt, or debris can obstruct the photocell's ability to sense light.

* Electrical wiring issue: A loose wire or faulty connection in the wiring could prevent the light from receiving the signal to turn off.

* Malfunctioning switch: The switch itself could be stuck in the "on" position.

Troubleshooting Steps:

1. Clean the photocell: Use a soft cloth or a brush to gently clean the photocell lens.

2. Check the wiring: Ensure that the wiring is secure and there are no loose connections.

3. Test the switch: Try turning the switch off and on multiple times to see if it's functioning properly.

4. Replace the photocell: If cleaning and checking the wiring doesn't solve the problem, the photocell may need to be replaced.

Note: If you're not comfortable working with electrical wiring, it's best to consult a qualified electrician.