Your car's engine has two main components; there is the block, which houses the pistons that move up and down to create the force needed to move your car forward, and there are cylinder heads, which cover the cylinders in which the pistons are kept. When cylinder heads get damaged they need repair or replacing, as they need to function properly in order to bring in oxygen that's is used to create the combustion inside the engine without allowing too much oxygen inside the engine block. 

Cylinder heads may get damaged and need service or replacing simply because they've worn down with time, or there may have been damage to the engine that also damages the cylinder heads. Note a few signs that the cylinder heads may need service or repair so you get this done as quickly as possible.

Constant overheating

The cylinder heads should allow in oxygen to create the combustion your engine needs to function, but they also open up to allow out heat from the engine. If your engine is constantly overheating this can be from a lack of coolant or broken fan, but it can also be because the cylinder heads are not opening as they should in order to allow heat to escape. If you've checked your engine's coolant levels and there is no problem with the fan or other such parts, note if the cylinder heads are cracked or not opening properly.

Discoloration and warping

Before cylinder heads crack, they usually discolor and warp. You may notice this warping on the sides so that they don't fit over the cylinders of the engine block as they should. Once cylinder heads crack, they will need replacing and they may let out oil or let in too much oxygen so that the combustion process is compromised and your engine misfires. Rather than wait for this to happen, have the cylinder heads replaced as soon as you see this warping or discoloration.

Sweet smell

Radiator fluid has a sweet smell but you should never be able to detect this smell as it should always be contained in the coolant system. If you can smell something sweet from your engine, the cylinder heads may be allowing radiator fluid or coolant to leak into the engine's block. If they're cracked or otherwise damaged, this can allow in that fluid and in turn, it gets warmer than it should and you can notice the odor. If there are no leaks in the radiator and you can smell this coolant, check the cylinder heads for cracks.