How to easily fix a no or low-flow at a faucet?

I am a professional plumber serving the San Diego area for more than 40 years. Over the years, I’ve responded to clients complaining about low water pressure at sink faucets, showers, or even that their toilet takes a long time to refill. My first response is to determine if this is a whole house problem or if it’s confined to a particular fixture. 
Let's address the situation where it’s a problem at a specific fixture, as opposed to a whole house issue (which we can discuss another time). Let’s say the complaint is that the bathroom sink faucet runs slower than usual. The first step is to check if water flow is restricted from both the hot and cold supply, or if it’s isolated to one supply only. Turn on the cold water and observe the flow rate, then turn off the cold supply and turn on the hot supply only to see if the same problem persists. If the low flow rate is the same, whether one or both supplies are on at the same time, it’s most likely that the aerator (a small screened fitting attached to the end of the faucet) is clogged. 
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The aerator is threaded into the faucet spout and can usually be removed easily, either by hand or, if needed, using appropriately sized pliers while protecting the finish with a rag to avoid scratches. Once removed, flush out the aerator by holding it upside down under the faucet spout and use the faucet's water to backflush it. This also allows you to check if your faucet spout is delivering full pressure. Without the aerator in place, you can also blow through the aerator, first from the outlet side, then from the inlet side.

Note: If the faucet spout still has low flow without the aerator in place, this suggests something inside it is restricting the flow, and most likely would require faucet removal for repair or replacement.

After cleaning the aerator, simply reinstall it, ensuring the rubber washer seal is in place and not deteriorated. Hand tightening is usually sufficient, but you may need to use pliers again to achieve the proper seal—avoid over-tightening.
This is generally a quick and easy fix, but as with any plumbing service, it’s important to know where to shut off the incoming water supply serving the house, just in case Murphy’s law kicks in. For the well-being of your house plumbing system, maintenance is vital to avoid unnecessary repairs, fixture replacement, or catastrophic plumbing failures. 

We recommend every home and property owner install the “Don Alarm, The Water Pressure Alert", to provide 24/7 monitoring of your plumbing water pressure. This way, you’ll know when your pressure is too high or too low and can take proper action to keep it within the recommended range of 50-60 psi.

Remember: High water pressure can lead to terrible consequences if allowed to continue without service. It could indicate that the water pressure regulator has failed or that your thermal expansion tank is no longer functioning. “You need to know!"