Here's What That Random Circle On Your Car's Dashboard Is For

You're sitting in your car, perhaps waiting at a red light, when your eyes wander across the instrument panel. Among the buttons, vents, and dashboard warning lights, there's a small unmarked circle near the windshield that you've never really noticed before. So, what is this circle? It doesn't light up, it doesn't click when you press it; so what's it actually for? That little circle is more important than you think. It's called a sunload sensor. While it might not look like much, it helps keep you comfortable in the cabin. 

The sunload sensor is typically located on the top of the dashboard, often near a defroster vent or speaker grill. As you might infer from the name, the sunload sensor constantly monitors the amount of sunlight hitting your vehicle's interior. But why does that matter? Surprisingly, this particular sensor has nothing to do with your car's headlights or dashboard brightness — that's another sensor entirely. Instead, the sunload sensor is tied to your vehicle's climate control system and helps in regulating your air conditioning.

Ever wonder how your car seems to know when the sun is blazing and cranks up the A/C on its own? That's the sunload sensor at work. Sunload sensors use a photodiode, a semiconductor that reacts to light by generating an electrical signal. The brighter the sunlight, the higher the resistance in the sensor, which lowers the voltage sent to the climate control system. In simpler terms, when the sun is beating down on the dashboard, the sensor detects that intensity and tells your car's A/C system to kick into high gear. How does it all come together?

The signals from the sunload sensor prompt your climate control system to increase the fan speed and adjust the blend doors, which are small flaps that regulate the mixing of hot and cold air before it reaches the vents. It's important to note that sunload sensors don't react to artificial light. So, if you try testing one in your garage under money-saving LED light bulbs, it might not respond at all. Sunload sensors are calibrated specifically for natural sunlight.

A properly functioning sunload sensor keeps your car's climate control in check. Besides measuring how much sunlight enters the car, the sunload sensors determine which side of the cabin is getting the most heat before automatically adjusting the airflow. This simplifies climate control adjustments, so the driver and passenger aren't stuck battling over the A/C setting.

If your car's A/C seems to have a mind of its own — blasting frigid air in one moment and barely keeping up with the heat in the other — it might be the result of a faulty sunload sensor. You might notice inconsistent cabin temperatures, where the A/C doesn't seem to respond correctly to the sun's intensity or adjustments to the controls. Lack of efficient cooling might also be a sign that your car's A/C needs charging. In some cases, a faulty sunload sensor might trick the system into thinking it's nighttime, causing the A/C to underperform.