Modern cars are full of incredibly ingenious engineering – and a lot of it goes completely unnoticed. Take, for instance, the frits on your windscreen: present on just about every automobile out there, and yet, unless they’ve been specifically pointed out to you, they may as well be invisible for how much attention we give them.

And equally if not more subtle is the solar, or sunload, sensor. It generally looks like a little round knob at the back of your dash – sometimes it’s circular, sometimes not; sometimes it sticks up, and sometimes it’s flush with the surface, but we promise you: it’s there, even if you’ve never noticed it.

“Sunload sensors are usually located at the top of the dash and are often mounted on a removable plate, speaker grill, or defroster vent,” confirms Canadian automotive aftermarket industry magazine Auto Service World. “There is often one on either side of the dash to accommodate differences in sunlight exposure.”

But what are they there for? Unsurprisingly, it’s to monitor how much sunlight is reaching the inside of the vehicle – but what might be slightly unexpected is why. It’s not, as you might think, to keep tabs on how bright it is outside, perhaps to activate the automatic headlights or brighten the dash display at night – there is a sensor for that, we know, but it’s not this one. 

Rather, the sunload sensor is part of the climate control system.

“Sunload sensors monitor the intensity of sunlight and adjust the HVAC [heating, ventilation, and air conditioning] control to improve the comfort level of the driver and passengers on sunny days,” Auto Service World explains. 

“Bright sunlight conditions can result in the fan speed increasing, and blend doors opening to improve cooling,” it adds.

Generally speaking, these sensors work using what’s known as a photodiode: a semiconducting diode which converts photon radiation – in this case, visible light, but they can also be sensitive to infrared, X-rays, gamma rays, you name it – into electrical current. However, as the light intensity increases, so too does the resistance of the diode, and the result is that fewer volts make it from the sensor to the car’s internal monitoring systems.

Like a well-oiled machine – hopefully literally – the control module then takes this drop in voltage as a signal to boost the A/C systems, keeping the driver from the horrible fate of boiling to death in an SUV.

And as anyone who’s been stuck in a car with a faulty HVAC system knows, that’s not totally hyperbolic. The sunload sensor has a very important job to do: “Sunlight entering a vehicle can account for up to 60 percent of the interior heat load that the system must overcome,” points out Auto Service World.

In other words, without this little blob on your dash, your car could end up absorbing up to two and a half times more heat than if it were there. Phew!