Direct answer: A frozen pipe under a sink is usually caused by cold air getting into the cabinet or wall cavity, especially on an exterior wall. Before you try to thaw it, look closely for a split, bulge, or active leak. If the pipe is intact, you can often thaw it with gentle warm air and the faucet slightly open.
Most likely: Most often, the freeze is in the supply line inside the sink cabinet or just behind it in an outside wall where insulation is thin and cold air is washing over the pipe.
Start simple and stay controlled. Figure out whether you have no water at the faucet, only one side frozen, or signs the pipe already cracked. Reality check: a pipe can thaw and start leaking minutes later, so keep watching it while it warms. Common wrong move: blasting one small spot with intense heat instead of warming the whole frozen section gradually.
Don’t start with: Do not start with a torch, heat gun on high, boiling water, or anything that can scorch the cabinet, melt plastic pipe, or turn a frozen pipe into a burst pipe.
The faucet barely drips or nothing comes out on either handle, usually after a hard freeze.
Start here: Check for a freeze in the cabinet, shutoff valves, and the wall behind the sink before looking farther down the house line.
Cold works but hot does not, or the other way around.
Start here: Trace that one under-sink supply line from the shutoff valve toward the wall and look for the coldest section, frost, or a slight bulge.
The sink is on an exterior wall, the cabinet feels like outside air, or there is a gap around pipes entering the wall.
Start here: Treat it like a local freeze first and warm the cabinet and exposed piping gently while checking for cracks.
Flow returned after temperatures rose or after warming the area, but now there is water under the sink or on the wall.
Start here: Stop using the faucet and inspect for a split supply tube, cracked shutoff valve body, or a burst section just inside the wall.
This is the most common setup: exterior-wall sink, little insulation, and a cabinet that traps cold instead of house heat.
Quick check: Open the cabinet and feel for a strong cold draft near the back panel, floor, or pipe penetrations.
The visible pipe may look normal, but the actual ice plug is often a few inches inside the wall where insulation is missing or compressed.
Quick check: Touch the pipe where it disappears into the wall and compare it to warmer sections in the cabinet.
If only hot or only cold stopped, the freeze is usually limited to that side's valve, riser, or short branch.
Quick check: See whether one shutoff valve and supply tube are much colder than the other side.
Copper can bulge and split, and plastic lines can crack or open at fittings once ice expanded inside them.
Quick check: Look for a hairline split, swollen section, loose fitting, or water appearing as the pipe starts to warm.
You want to separate a local under-sink freeze from a wider water supply issue before you start warming cabinets and opening walls.
Next move: If other fixtures work normally and this sink is the problem, you are likely dealing with a local frozen section under or behind the sink. If multiple fixtures are dead, or a whole hot or cold branch is out, the freeze may be farther upstream than the sink cabinet.
What to conclude: A local freeze is often manageable with careful warming. A wider freeze needs a broader search and may be better handled by a plumber, especially if the frozen section is hidden.
A pipe that already cracked can stay quiet while frozen, then leak hard as soon as the ice plug melts.
Next move: If everything stays dry and the pipe looks intact, you can move to controlled thawing. If you find a crack, split, or active drip, shut off water to that branch or to the house and plan for repair instead of thawing further.
What to conclude: No visible damage means the pipe may thaw safely. Visible damage means the freeze already turned into a leak problem.
Slow, even warming is the safest way to melt the ice plug without damaging the pipe, cabinet, or nearby finishes.
Next move: If water starts to spit, then flow steadily, keep warming for several more minutes and keep watching for leaks. If the exposed pipe warms up but flow does not return, the freeze is probably deeper in the wall or farther upstream.
The most important moment is right after flow returns, because that is when a hidden split often shows itself.
Next move: If the pipe stays dry and flow is normal, you likely avoided a burst and can move on to prevention. If any leak appears, stop using that sink and repair the damaged section or call a plumber if the break is in the wall.
If you only thaw the pipe and walk away, the same cabinet or wall cavity will freeze again on the next hard cold snap.
A good result: If the cabinet stays warmer and the pipe no longer freezes during cold weather, you solved the real problem instead of just the symptom.
If not: If the line still freezes even after air-sealing and insulation, the vulnerable section is likely hidden in the wall and needs a more permanent correction.
What to conclude: Prevention is usually about stopping cold air and protecting the exposed branch. Repeated freezing points to a layout or insulation problem beyond the cabinet.
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A frozen pipe usually gives you little or no water with no leak yet. A burst pipe may still be frozen at first, then start dripping or spraying as it thaws. Look for a split, bulge, damp fitting, or water showing up at the wall opening or cabinet floor.
It is better not to. Hot or boiling water can shock some materials, make a mess in the cabinet, and does not warm hidden sections well. Gentle moving warm air is safer and gives you more control.
That usually means the hot supply line or shutoff is the one sitting in the coldest spot. One side freezing by itself is common when the lines are routed differently inside the cabinet or wall.
Yes. Open the affected faucet slightly. That relieves pressure and lets you know when the ice plug starts to melt because you will hear sputtering or see flow return.
That points to a cold-air or insulation problem, not just bad luck. Seal drafts, insulate the exposed pipe, and if the freeze is inside the wall, have a plumber look at rerouting or another permanent fix.
Usually no. A space heater in or aimed into a cabinet can overheat finishes, wiring, stored items, or plastic parts. A hair dryer used carefully and kept moving is the safer homeowner option for a local thaw.