i installed it but when i flip the switch to filtered, no water comes out at all. unfiltered works fine.

Diagnosis

The total lack of water flow when switching to the "filtered" position on a PUR PLUS PFM400H indicates a complete mechanical obstruction in the water path. Because the "unfiltered" setting works, we know the faucet's main valve is functioning and water is reaching the diverter assembly. The blockage is located specifically within the filter housing or at the interface between the cartridge and the diverter valve. The most common cause is the failure to remove the protective shipping cap from the top of the replacement cartridge. This cap is a hard plastic seal designed to prevent contamination during transit; if left in place, it acts as a plug, preventing water from entering the filter media. Alternatively, if the cap was removed, the issue is likely an "air lock" or a misalignment of the O-ring seal, where the cartridge is not seated deep enough to engage with the diverter's intake port, effectively blocking the flow of water into the filtration chamber.

How to Fix It

Follow these precise steps to clear the blockage and ensure a proper seal:

1. Turn the diverter switch to the "unfiltered" position and turn off the faucet completely.
2. Unscrew the clear plastic filter housing by rotating it counter-clockwise. Be prepared for a small amount of residual water to leak out.
3. Carefully lift the filter cartridge out of the housing.
4. Examine the top of the cartridge (the protruding nipple). Look for a small, clear or blue plastic cap. If present, grip it with your fingernails or a pair of needle-nose pliers and pull it straight off. Ensure no plastic fragments remain on the nipple.
5. Inspect the black rubber O-ring at the top of the cartridge. Ensure it is seated flat and hasn't been pinched or twisted during the initial installation.
6. Re-insert the cartridge into the housing, pressing firmly upward to ensure the nipple is fully engaged with the diverter valve.
7. Screw the housing back onto the faucet mount. Tighten it hand-tight; do not use a wrench, as over-tightening can crack the plastic threads or warp the O-ring.
8. Turn the faucet on in the "unfiltered" position for 30 seconds to clear any air from the mount, then flip the switch to "filtered" and let the water run for 2-3 minutes to flush out carbon fines (black specks).

If That Didn't Work

• Check for Air Lock: If the cap was removed but water still won't flow, you likely have a pocket of air trapped in the diverter. Remove the housing and the filter. Turn the faucet on in the "filtered" position for 5 seconds without the filter attached to "prime" the valve. Reinstall the filter and try again.
• Inspect the Diverter Valve: Remove the entire PFM400H assembly from the faucet aerator. Check the internal diverter switch for debris or mineral buildup (calcium deposits). If the switch feels "mushy" or doesn't click firmly into place, use a toothpick to clear any grit from the valve housing.
• Test with a Different Cartridge: While rare, some cartridges have a collapsed internal mesh or a manufacturing defect where the carbon block is too dense for water to permeate. If you have a spare cartridge, swap it in. If the second cartridge works, the first one was defective.
• Verify Aerator Compatibility: Ensure the mount is threaded perfectly onto your faucet. If the mount is cross-threaded, it can put pressure on the diverter switch, preventing it from fully shifting into the "filtered" position even if the lever looks correct.

Heads Up

• Flush Period: Always run the filtered water for at least 3 full minutes when installing a new cartridge. This removes trapped air and loose carbon particles that can cause a "sputtering" flow.
• Avoid Over-tightening: The PFM400H uses plastic threading. Using pliers to tighten the housing can strip the threads, leading to permanent leaks that cannot be fixed without replacing the entire unit.
• Replacement Cycle: Change the filter every 40 gallons or every 2 months. A clogged filter will increase pressure inside the housing, which can eventually lead to a hairline crack in the clear plastic casing.
• Temperature Warning: Do not run boiling hot water through the filter. High temperatures can damage the carbon block and the plastic seals, potentially causing the filter to leak or fail prematurely.