my new PUR PLUS faucet filter is leaking water from the top where it connects to the faucet, i tightened it as much as i could.

Diagnosis

The leak occurring at the top connection of the PUR PLUS PFM400H is rarely caused by insufficient tightening; in fact, over-tightening often exacerbates the issue by distorting the sealing surfaces. The root cause is typically a failure in the compression seal between the faucet adapter and the faucet aerator threads. This happens when the black rubber O-ring or gasket is either pinched, misaligned, or missing entirely from the adapter housing. When you tighten the unit "as much as you could," you may have actually compressed the rubber gasket past its sealing point, causing it to buckle or "mushroom" out of the groove. This creates a gap where water can bypass the seal under pressure. To confirm this, wipe the connection dry with a paper towel and run the water on low; if the water beads specifically at the junction where the plastic adapter meets the metal faucet threads, the gasket is compromised.

How to Fix It

Follow these precise steps to reseat the seal and stop the leak:

1. Turn off the water supply and unscrew the PFM400H filter unit from the faucet adapter by rotating the housing counter-clockwise.
2. Remove the adapter from your faucet. Inspect the interior of the adapter's threaded collar. Locate the black rubber gasket. If it is missing, you cannot use the unit; if it is present, remove it carefully using a toothpick or small screwdriver to inspect for tears or twists.
3. Clean the threads of your faucet and the inside of the adapter with a damp cloth to remove any mineral deposits or grit that could prevent a flush fit.
4. Reinsert the rubber gasket into the adapter. Ensure it sits perfectly flat within the recessed groove and is not pinched against the plastic wall.
5. Thread the adapter onto the faucet by hand. Turn it clockwise until it is snug. Do not use pliers or wrenches; the PFM400H is designed for hand-tightening only. Over-torquing will crack the plastic housing.
6. Attach the filter cartridge to the adapter. Tighten until you feel firm resistance, then give it one final quarter-turn.
7. Run the water in "filtered" mode for 60 seconds to flush any trapped air, which can sometimes cause initial pressure spikes that lead to leaks.

If That Didn't Work

• Check for Aerator Compatibility: Ensure your faucet has standard male threads. If your faucet has a "hidden" aerator (integrated into the spout) or an unconventional thread pitch, the PFM400H adapter will not create a vacuum seal. You may need to remove the original factory aerator completely before installing the adapter.
• Inspect for Debris: Small pieces of Teflon tape or old plumber's putty left over from previous faucet repairs can prevent the gasket from seating. Use a needle-nose plier to clear the faucet threads of any white or yellow tape remnants.
• Replace the Gasket: If the rubber gasket appears flattened or has a permanent "dent" from over-tightening, it has lost its elasticity. You will need to source a replacement 1/2" or 5/8" rubber washer (depending on your faucet size) from a hardware store to replace the factory PUR gasket.
• Verify Adapter Alignment: If the adapter is cross-threaded even slightly, water will leak from the top. Unscrew it completely and restart the threading process slowly, ensuring the adapter enters the faucet threads perfectly perpendicular.

Heads Up

• Avoid Tools: Never use a wrench or pliers on the PFM400H plastic components. The plastic is prone to stress fractures, and a hairline crack in the adapter will cause a permanent leak that cannot be fixed.
• Mineral Buildup: Every 3 months, soak the adapter in a 50/50 solution of white vinegar and water for 30 minutes to remove calcium buildup that can degrade the rubber seal.
• Pressure Warning: If you have an extremely high-pressure home water system, you may notice "seeping" during high-flow usage. Ensure the diverter switch is fully clicked into the "filtered" or "unfiltered" position; leaving it in between can cause turbulence and leaks.
• Cartridge Seating: Ensure the filter cartridge is pushed fully into the adapter before twisting. If there is a gap between the cartridge shoulder and the adapter, the internal seal will fail.