Direct answer: If cold water works but the hot side does not, the problem is usually local to that faucet: a partly closed hot shutoff, debris packed into the hot side of the aerator after plumbing work, or a faucet cartridge that is stuck or blocked.
Most likely: On most kitchen and bath faucets, start with the hot shutoff under the sink and the aerator at the spout before you assume the faucet itself is bad.
First figure out whether this is one-faucet-only or a house-wide hot water problem. That one split saves a lot of wasted time. Reality check: a faucet that suddenly lost hot flow right after shutoff work or a water heater service call often just has loosened scale or debris lodged in the hot side. Common wrong move: forcing the handle harder when the cartridge is binding can crack trim or strip the handle screw.
Don’t start with: Do not start by replacing the whole faucet or the water heater. If other fixtures still get hot water, stay at this faucet first.
The faucet runs normally on cold, but when you move to hot the flow drops to nothing or almost nothing.
Start here: Check whether the hot shutoff under that sink is fully open, then remove and inspect the faucet aerator for debris.
You still get some hot water, but the stream is noticeably weaker than cold.
Start here: Suspect debris in the faucet aerator or a faucet cartridge that is restricted on the hot side.
Water flows on the hot side, but it stays lukewarm while other fixtures get hotter.
Start here: Run the faucet long enough to rule out a long branch line, then look for a handle limit stop setting or a cartridge that is not opening fully to hot.
Hot water was fine before a valve was closed, supply lines were disturbed, or work was done elsewhere in the house.
Start here: Look for a hot stop valve left partly closed and flush the faucet with the aerator removed to clear loosened mineral debris.
This is the fastest, most common one-faucet cause, especially after someone worked under the sink or stored items bumped the valve.
Quick check: Open the cabinet, find the hot side stop, and make sure the handle or stem is fully open without forcing it.
When sediment breaks loose, it often packs into the aerator and shows up as weak or no flow on one side first.
Quick check: Unscrew the faucet aerator, rinse the screen, and test the faucet briefly with the aerator off.
A bad faucet cartridge can let cold flow normally while the hot side barely opens or not at all.
Quick check: If the hot stop is open and flow is still poor with the aerator removed, the faucet cartridge moves to the top of the list.
If both hot and cold act odd, or several fixtures lost hot water, the trouble is farther upstream than the faucet body.
Quick check: See whether nearby sinks, tubs, or showers still get normal hot water before taking the faucet apart.
You do not want to tear into a faucet when the real issue is the water heater, a branch shutoff, or a wider hot-water interruption.
Next move: If other fixtures have normal hot water, you have narrowed this down to the faucet, its shutoff, or the local hot supply under the sink. If hot water is missing or weak at multiple fixtures, this page is no longer the right repair path.
What to conclude: A one-faucet problem is usually a shutoff, aerator, or cartridge issue. A whole-house or multi-fixture problem points away from the faucet.
A partly closed hot stop can mimic a bad faucet, and it is the least destructive thing to check.
Next move: If hot flow returns, the fix was a local supply restriction and you can stop here. If the valve is open and the hose is not kinked, move to the aerator and flushing check.
What to conclude: A good cold side with a restricted hot side under the sink usually means the faucet is not getting enough hot water to begin with, or debris is trapped farther downstream.
Debris at the faucet outlet is common after plumbing work, water heater flushing, or old galvanized or scaled piping disturbance.
Next move: If hot flow improves with the aerator off or after cleaning, the blockage was at the outlet and the faucet itself is probably fine. If hot flow is still weak or dead even with the aerator removed, the restriction is inside the faucet or just before it.
Once the hot stop is open and the aerator is ruled out, the cartridge becomes the most likely faucet-side part causing no hot flow or poor hot mixing.
Next move: If you find a mis-set hot-limit stop and correct it, full hot water may return without replacing parts. If there is no limit-stop issue and the hot side still will not flow properly, the faucet cartridge is the supported repair path.
By this point you should know whether you have a simple outlet blockage, a local shutoff issue, a cartridge problem, or something beyond the faucet.
A good result: If the faucet now has normal hot flow and reaches expected temperature, the repair is complete.
If not: If a new cartridge does not restore hot flow, stop buying faucet parts and move upstream to the shutoff valve, supply line, or branch piping.
What to conclude: A successful repair confirms the fault was inside the faucet assembly. No improvement after the supported faucet fixes means the faucet was not the only problem.
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If only one faucet is affected, the usual causes are a closed hot shutoff under the sink, debris clogging the faucet aerator after plumbing disturbance, or a faucet cartridge that is stuck or blocked on the hot side.
Usually no. If the water heater were the problem, you would normally notice it at multiple fixtures. One bad faucet almost always points to a local faucet or under-sink supply issue.
Closing and reopening valves can shake loose scale and sediment. That debris often ends up in the faucet aerator or inside the faucet cartridge, especially on the hot side.
Not first. Check the hot stop, hose, and aerator before buying anything. If those are fine, replacing the faucet cartridge is the usual next move. Whole faucet replacement makes more sense when the body is cracked, badly corroded, or parts are no longer available.
Run it long enough to clear cooled water from the pipe first. If other fixtures get hotter and this one does not, look for a handle limit stop setting on a single-handle faucet or a cartridge that is not opening fully to the hot side.
Yes, for mineral buildup on the removable aerator parts, a short soak in plain white vinegar is usually fine. Rinse well and reassemble the parts in the same order. Do not soak decorative faucet trim or mix cleaners.