Direct answer: When a faucet temperature will not adjust, the usual cause is a stuck or worn faucet cartridge, a loose handle that is no longer turning the stem correctly, or one side of the water supply not reaching the faucet at all.
Most likely: Start by checking whether the handle actually moves the faucet stem and whether both hot and cold shutoffs under the sink are fully open. If both supplies are present and the handle still gives little or no temperature change, the faucet cartridge is the most likely fix.
First separate the lookalikes: a faucet that is stuck on one temperature is different from a faucet with weak flow on one side, and both are different from a house-side hot or cold water problem. Reality check: if the faucet worked normally for years and then slowly lost temperature control, the cartridge is far more likely than a sudden mystery in the plumbing. Common wrong move: forcing the handle harder usually strips the handle adapter or cracks trim before it fixes anything.
Don’t start with: Do not start by buying a whole new faucet. Most of these calls turn out to be a handle issue, a partly closed stop valve, mineral buildup, or a bad faucet cartridge.
The handle moves, but the faucet never gets warm or only turns slightly lukewarm.
Start here: Check whether the hot shutoff under the sink is fully open and whether other nearby faucets get normal hot water. If they do, focus on this faucet's handle and cartridge.
The faucet comes out hot and barely cools down even when you swing the handle toward cold.
Start here: Confirm the cold shutoff is open and the cold side has pressure. If the cold supply is present elsewhere and weak only here, the faucet cartridge or inlet passages are likely restricted.
You can get warm water, but not a full hot-to-cold range like before.
Start here: Look for a loose handle, mineral buildup around the cartridge, or a faucet cartridge that is no longer mixing smoothly.
The handle is hard to move, spins oddly, or moves without much change in water temperature.
Start here: Remove the handle cap or set screw cover and check whether the handle is loose, stripped, or not fully engaging the faucet stem.
This is the most common reason a single-handle faucet gets stuck hot, stuck cold, or loses most of its mixing range. Inside wear or mineral scale keeps the ports from lining up correctly.
Quick check: With both shutoffs fully open, move the handle slowly from full cold to full hot. If flow stays steady but temperature barely changes, the faucet cartridge is the leading suspect.
If the handle is not gripping the stem, your hand is moving trim but not fully rotating or lifting the actual mixing mechanism.
Quick check: Watch the stem while the handle is off or loosened. If the handle moves more than the stem, or the set screw will not hold, the handle or adapter is damaged.
A faucet cannot mix properly if one side is starved. A half-closed stop valve or kinked local supply hose can make the faucet act like the cartridge failed.
Quick check: Open the cabinet and verify both hot and cold shutoffs are fully open. Compare flow and temperature at another faucet nearby to see whether one supply is missing only at this fixture.
Hard-water scale can choke one side of the mix path or make the handle feel stiff. Sometimes the faucet still runs, but the temperature range gets narrow.
Quick check: If the handle feels gritty or stiff and the aerator has visible scale, remove the aerator and retest. If temperature control improves only slightly, buildup inside the faucet cartridge area is still likely.
You do not want to tear into the faucet if the house is not delivering hot or cold water to this location in the first place.
Next move: If another faucet also lacks hot or cold water, the problem is upstream and not mainly this faucet. If other faucets work normally and both local shutoffs are open, stay focused on this faucet.
What to conclude: A temperature problem at one faucet usually points to the handle, cartridge, aerator, or local supply connection rather than the whole plumbing system.
A loose or stripped handle can mimic a failed cartridge, and it is the fastest thing to rule out.
Next move: If tightening the handle restores normal temperature control, you found the problem. If the handle is solid but the faucet still stays hot, cold, or lukewarm, the cartridge or one local supply path is more likely.
What to conclude: A handle that slips or free-plays means the faucet was never getting full stem movement. A solid handle with poor mixing points deeper into the faucet.
Scale at the faucet outlet will not usually cause a full temperature lock, but it can hide a weak hot or cold side and make diagnosis muddy.
Next move: If the faucet suddenly has a normal temperature range with the aerator cleaned, reinstall it or replace the faucet aerator if it is damaged. If the faucet still will not mix correctly, the problem is likely inside the faucet body at the cartridge or local inlet passages.
Once the handle and supplies check out, the cartridge becomes the main suspect. This is where you confirm whether it is stuck, scaled, or worn out.
Next move: If cleaning and reinstalling a lightly scaled cartridge restores smooth full-range mixing, you may be done. If the cartridge is damaged or cleaning does not restore control, replace the faucet cartridge. If the handle itself was stripped, replace the faucet handle instead.
Once you know whether the handle or cartridge is the fault, the finish-the-job move is straightforward: replace that part, reassemble carefully, and verify the faucet now mixes normally.
A good result: If the faucet now moves smoothly and gives a full hot-to-cold range without leaks, the repair is complete.
If not: If the new cartridge and a solid handle still do not restore temperature control, the faucet body may be internally damaged or one local supply path may be blocked. At that point, a plumber or full faucet replacement is the cleaner next move.
What to conclude: A confirmed part replacement should restore normal mixing. If it does not, the remaining problem is usually in the faucet body or local supply path, not another random part guess.
Repair Riot may earn a commission from qualifying purchases, at no extra cost to you.
Usually the cold side is not reaching the faucet, the handle is not turning the stem correctly, or the faucet cartridge is stuck in a hot-biased position. Check the cold shutoff under the sink first, then the handle, then the cartridge.
Yes. A worn or scaled faucet cartridge can narrow the mixing range so you only get lukewarm water instead of a full hot-to-cold sweep. That is especially common when the handle still feels normal but the temperature range keeps shrinking.
Not first. A whole faucet replacement is usually unnecessary until you confirm the faucet body is cracked, badly corroded, or still will not mix correctly after a proper cartridge or handle repair.
That usually means the handle is loose on the stem or the faucet cartridge inside is no longer moving the internal ports the way it should. A stripped handle is quick to spot. A solid handle with poor mixing points more toward the cartridge.
Not usually by itself, but a clogged faucet aerator can make one side seem weak and muddy the diagnosis. It is worth cleaning because it is easy, but a faucet stuck hot or cold usually comes back to the supply side, handle, or cartridge.