Step-by-step fix
Step 1: Make sure the cold supply line is really the problem
- Open the cold side of the faucet and note the symptom: visible dripping under the sink, corrosion on the braided line or nuts, a bulge in the line, or weak cold flow that seems to improve when the line is moved or disconnected.
- Dry the line, the shutoff valve, and the faucet connection with a towel.
- Turn the cold water on for a moment and watch where fresh water appears.
- Confirm the leak or restriction is at the cold supply line itself, not from the shutoff valve body, faucet, or drain parts nearby.
If it works: You have a clear reason to replace the cold supply line and know which connection points it runs between.
If it doesn’t: If you cannot tell where the water is coming from, dry everything again and place a dry paper towel around each connection one at a time while the cold water runs briefly.
Stop if:- Water is coming from the shutoff valve body or from inside the wall.
- The cabinet floor, wall, or sink mounting area is soft, swollen, or badly water-damaged.
- The faucet connection is cracked, stripped, or too corroded to reconnect safely.
Step 2: Shut off the cold water and set up the work area
- Place a towel and small bucket under the cold shutoff valve and supply line.
- Turn the cold shutoff valve clockwise until it stops. Leave the hot valve alone unless it also leaks or gets bumped easily.
- Open the faucet cold side to relieve pressure and let the remaining water drain out.
- Use the flashlight to confirm you can reach both ends of the line comfortably.
If it works: The cold side is depressurized and the area under the sink is protected from drips.
If it doesn’t: If the cold shutoff valve does not fully stop the water, close the home's main water supply before removing the line.
Stop if:- The shutoff valve will not close and the main water shutoff is not available or does not work.
- The valve stem starts leaking heavily when you turn it.
Step 3: Remove the old cold supply line
- Hold the shutoff valve steady if needed and loosen the supply line nut at the valve with the adjustable wrench.
- Let the remaining water drain into the bucket.
- Loosen the upper end of the line from the faucet connection.
- Remove the old line and compare both ends, the overall length, and the thread style to the new line before installing anything.
If it works: The old line is out and you have confirmed the new line matches both connection points and is close to the right length.
If it doesn’t: If the new line does not match, take the old line with you and buy one with the same end sizes and connection style in a length that reaches without stretching or looping sharply.
Stop if:- A connection nut is seized so badly that the faucet shank or shutoff valve starts twisting with it.
- The shutoff valve outlet or faucet threads are damaged, split, or badly corroded.
Step 4: Install the new supply line without twisting or forcing it
- Start the upper faucet connection by hand first so the threads engage cleanly.
- Start the lower shutoff valve connection by hand next.
- Route the line in a smooth curve with no sharp bend, kink, or rubbing against a rough cabinet edge.
- Tighten both ends snugly with the wrench after hand-threading. Do not overtighten; the goal is a firm seal, not crushed fittings.
- Make sure the line is not under tension and does not pull sideways on either connection.
If it works: The new cold supply line is fully connected, properly routed, and seated squarely at both ends.
If it doesn’t: If a nut will not thread smoothly by hand, back it off and realign it before tightening again.
Stop if:- The line only reaches by stretching tight or bending sharply.
- A fitting cross-threads or will not seat flat against the connection.
Step 5: Turn the water back on and check for leaks
- Close the faucet.
- Slowly open the cold shutoff valve while watching the lower connection first, then the upper connection.
- Once the valve is open, run the cold water at the faucet for a minute to flush air and confirm normal flow.
- Wipe both connections dry and check again for fresh moisture.
- If needed, tighten a leaking connection slightly, then dry it and recheck.
If it works: The cold water runs normally and both ends of the new line stay dry under pressure.
If it doesn’t: If a connection still seeps after a small retightening, shut the water back off, disconnect that end, inspect for damage or misalignment, and reconnect it carefully.
Stop if:- A steady drip continues after careful realignment and slight tightening.
- The shutoff valve itself now leaks from the stem or body.
- The new line balloons, rubs hard against something sharp, or shifts when pressure comes on.
Step 6: Confirm the repair holds in normal use
- Use the sink several times over the next few hours with normal cold-water flow.
- Check under the sink after each use and again later when the line has been under pressure for a while.
- Run both hot and cold together once to make sure cabinet vibration or faucet movement does not disturb the new line.
- Leave a dry paper towel under the connections for a final leak check.
If it works: The paper towel stays dry and the cold side works normally without drips, kinks, or pressure loss.
If it doesn’t: If the area stays dry but cold flow is still weak, the restriction may be in the shutoff valve, faucet aerator, or faucet cartridge rather than the supply line.
Stop if:- You find recurring moisture that is not clearly from the new line.
- Water damage continues to appear inside the cabinet or wall even though the new connections are dry.
Replacement Parts
Repair Riot may earn a commission from qualifying purchases, at no extra cost to you.
FAQ
How do I know the cold supply line is bad and not the faucet?
If water appears on the braided line, at either line nut, or the line looks corroded, bulged, or kinked, the supply line is a good suspect. If the line stays dry but flow is still poor, the problem may be in the shutoff valve, aerator, or faucet cartridge instead.
Do I need thread tape on a bathroom sink supply line?
Usually no for the common compression-style or faucet supply connections used on sink lines. Those fittings seal at the connection surfaces, not on the threads. Match the replacement line to the original connection type and tighten it correctly.
What length supply line should I buy?
Use a line that reaches both connections in a gentle curve. It should not be stretched tight, and it should not need a sharp loop to take up extra length. Matching the old line is usually the easiest starting point.
Can I replace just the cold line and leave the hot line alone?
Yes, if only the cold line is leaking, restricted, or damaged. If the hot line is the same age and shows corrosion or wear too, many homeowners replace both while they are already under the sink.
Why is my new supply line still leaking at the nut?
The most common causes are a mismatched fitting, cross-threading, a connection that was not started by hand, or a damaged valve or faucet thread. Shut the water off, disconnect that end, inspect it, and reconnect it squarely before tightening again.