New kitchen faucet worked for 4 weeks, now the water flow is very low

There are several of things that could cause that but the first thing I would do is unscrew the little screen thing at the tip of the faucet.
If you then get good flow, clean it and put it back on.

water flow on my kitchen faucet is very low YouTube

The hot side of the faucet is most likely plugged with something that came up thru the water system. The average thankless waterheater requires at least 1/2 to 3/4 gpm flow rate for the unit to sense flow and turn on. If there is not enough flow it will not fire and heat the water. Not sure about this particular unit but a few require at least one full gallon per minute flow. So chances are that if you fix the faucet, you fix the hot water issue at the same time

I had the same issue with an American Standard faucet that I installed new 6 months ago. The diverter can be the issue if you have a sprayer attachment. Try unscrewing the sprayer head from the hose it's attached to. Now run the water full pressure with the hose pointed into the sink, obviously :~). After running the water for about 1 minute, connect the sprayer head back onto the hose, make certain the flat rubber washer and ring gaskets are seated in their grooves properly, and tighten until you have no water leaking from the sprayer head/hose connection. Now try the water through the faucet. If the problem was a temporary diverter issue, your pressure should be fine now. Good luck

1) Guesses: Sounds like a bad single-handled faucet cartridge, or low water flow at faucet, or clogged water filter.
Replace faucet cartridge, clean the aerator, and/or clean sediment out of pipes below sink, and clean water filter at tankless unit each month.

2) For exact measure of problem:
Typical faucet can have 2 gallon per minute flow rate.
2 gpm flow rate means faucet fills 1 gallon container in 30 seconds. Test faucet using 1 gallon container and a cooking timer to get flow rate.
Clean aerator to improve flow rate. Compare kitchen flow rate with other faucets in home.
http://waterheatertimer.org/Low-hot-water-pressure.html

Most homes have 1/2" diameter pipe running to kitchen faucet.
100 feet of 1/2" diameter pipe holds 1.2 gallons of water. This assumes no sediment build-up inside pipes.
http://waterheatertimer.org/Water-heater-recirculation-system.html

Assuming only hot is turned on: If faucet is 100 feet away from water heater, and pipe diameter is unobstructed 1/2" diameter, and faucet has 2 gallon per minute water flow, then hot water should arrive in 36 seconds.
50 feet away, and hot water should arrive in 18 seconds.

If kitchen has single handled faucet, and the handle is raised in the middle, then equal amounts of hot and cold water are drawn at same time. This doubles amount of time for hot water to arrive.

If single handle faucet has a bad cartridge, then faucet can have crossover.
Crossover means that cold water can enter the faucet even if faucet is turned to hot.
Bad single-handled faucet cartridge can delay or stop the arrival of hot water.
http://waterheatertimer.org/Crossover.html

Tankless require maintenance including cleaning combustion parts yearly, de-lime heat exchanger frequently, and clean water filter each month.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

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Remove and clean the Price Pfister aerator. Turn the water on after you remove the aerator and check the flow-rate. Many new Lavatory and kitchen faucets are now equipped with Eco friendly water saver aerators. if the flow is normal without the aerator, the faucet is OK. if you want more water flow from your lavatory faucet, you will need to buy one of the old style high flow aerators from your hardware store. If you remove the aerator and still get low pressure from both hot and cold faucets, you may need to replace the faucet. Call the Price Pfister warranty phone number that came with your faucet.

One thing you could try is to remove the aerator at the end of the spout on the faucet, Turn water on and see if pressure is any better. If so the aerator is got something in it restricting the flow.
You also can remove the dime shaped flat in the aerator to increase water flow. It looks like a dime with a hole about 1/8" in center to restrict the flow to about 1.6 gpm. Remove it and reinstall aerator and check water flow

Most of the times when cold and hot side of faucet.It would be the first thin to check being you have good water pressure at other faucets. They catch debris and the flow slows down from the faucet's. Check out supply lines and faucet for a blockage if this doesn't take care of the problem.

Check out this web sire for a parts breakdown of parts in faucet. Check out installation and rough in sheet for information on faucet below.
http://www.needplumbingsupplies.com/Kohler-Kitchen-Sink-Faucets.asp?id=3628

yes it should have a flow restrictor in the end of the spout. You might need to clean it out

when you installed did any teflon tape or debris get inside? you can remove and reverse flush with hose and it may clear the blockage

This could be a couple of things.

1. First remove the aerator from the wand and make sure you have plenty of flow with it off.
2. If there is flow clean or replace the aerator.
3. Remove the wand and look between the hose connection and the wand there should be a screen between them. Remove the screen and clean or replace.
4. Make sure hose is kinking under the sink.

If you need a diagram of the faucet with part numbers here is the link:

Delta 470

Hope this helps!

Ihad the same problem. It is not the screen. What you need to do is get a spray diverter the old one disintergated and is blocking the flow. You have to take the collar off and underneath is the diverter. This is after you take the ball and rings out.