You’ve probably heard the warning many times: don’t let your car run out of gas because it will damage it. But why is this true? And when can this happen? In his YouTube video, mechanical engineer Jason Fenske from Engineering Explained talks about what happens when your car is running low on fuel or when the gas tank is empty.

He identifies four things that are required for an engine to run:

1. Fuel
2. A spark
3. Compression
4. Oxygen

Fuel tops this list for a reason. If there is no fuel, no combustion will occur. The engine won’t function and will shut off.

Fenske focuses on three essential car components that could be damaged if your car runs out of gas: the fuel pump, the engine, and the catalytic converter.

The fuel pump

The fuel pump’s job is to move fuel from the fuel tank through a fuel filter. Then it’s sent to the front of the car where it’s introduced into the intake manifold by way of fuel injectors.

The fuel pump sits upright in the fuel tank and has a suction filter attached at the bottom that keeps out debris. This filter can get clogged up, which forces the fuel pump to work harder. This is more likely to happen when you’re about to run out of gas.

If your car is idling and your fuel tank is low, the fuel pump draws fuel from the bottom of the gas tank. Eventually, the car runs out of fuel and, at that point, the fuel pump will start to pull up air. This component is designed to cool and lubricate itself as it functions. It starts to heat up if it’s pumping only air. The engine will shut off and you’ve sidestepped damage because the car is not in motion.

But aggressive braking, accelerating, or cornering while you’re driving on a low tank of fuel can hurt the fuel pump. Baffles on the pump help to pull up the fuel while keeping out air. Rougher driving causes a low level of fuel to slosh around the tank, making it less available to be pulled in by the fuel pump. It draws in more air and is much more likely to fail.

With care, the fuel pump is durable and can last the life of your car. Fenske recommends that you check your owner’s manual to find out if a low fuel level might cause overheating for your car’s fuel pump.

The engine

When your car is about to run out of gas, the fuel may go into one cylinder but not another, causing the engine to misfire. What’s happening is that combustion is occurring only in the cylinder that has fuel. While this is a rare occurrence, it can cause harm to your engine.

A more common problem is running your car with a wide-open throttle when the fuel level is low. The fuel pump isn’t getting adequate fuel moving through it, and your car’s onboard computer is telling it to run on an air-fuel ratio of 12:1 or 11:1. With so little fuel in the tank, the fuel pump can’t supply that much fuel.

In this case, the air-fuel ratio is more like 14:1, making it run much hotter due to this less rich mixture. The result of a mixture with much more air than what is beneficial is engine knocking.

Your car’s computer attempts to find a specific air-fuel ratio to prevent damage. But there isn’t enough fuel for the engine to run at that ratio and increases the chance of harm to the engine. The easiest solution is to avoid running your car at high rpm when you’re about to run out of gas.

The catalytic converter

The catalytic converter is designed to be robust. When it removes pollutants from the exhaust caused by combustion, it handles a range of air-fuel ratios and exhaust temperatures in your car.

But if excess fuel flows through it, it can sustain damage. It overheats quickly if too a lean mixture of air and fuel is sent through it. And too much heat can damage this component.

Another problem is having a misfire caused by the spark that triggers combustion. If the spark fails to fire, all of the fuel passes through the catalytic converter at once.

If you’re about to run out of gas, a misfire indicates that less fuel will be passing through the catalytic converter. The fuel mixture going to the catalytic converter has too much air, which could cause overheating. But this situation presents a lower risk of damaging the catalytic converter since there isn’t much fuel in the gas tank and it happens in such a short amount of time.

To prevent damage to any of these components, it’s crucial to avoid running out of gas or running the tank low repeatedly. To do so means that you’re increasing the likelihood of causing damage to your car. It’s also a good idea to check your car’s owner’s manual to find out what the minimum amount of fuel your car needs to protect the engine, fuel pump, and catalytic converter.