Leaky faucet - how to fix

[Bodrey]

The faucet on my bathroom vanity is a Moen and looks like this...

https://ibb.co/N26nhCQJ

Recently, I began noticing water pooling around the base of the faucet when I ran it. I did a search and it suggested that the faucet aerator may be to blame. So, I took it off and soaked the gasket in rubber rejuvenator overnight and put it back on the next day. The problem seemed to be resolved. However, just yesterday I reached into the bottom of the vanity to pull out a new bar soap and discovered that the tops of many of the boxes are water saturated. That's when I realized the faucet had a small leak and was dripping down onto the soap boxes.

I observed the underside of the faucet while running water and it appears that the leak is coming directly from the faucet at the mounting hole (see pic below) and not the water supply line. It also seems to be leaking only from the cold side.

https://ibb.co/JFnz87Z8

I just replaced the cartridge thinking that it would resolve the issue and although the faucet operates much more smoothly now the leak persists. When I had the old cartridge out I inspected the top of the valve body and didn't see any sign of mineral buildup (i have hard water). Is it possible that the valve body is just worn out or that there's another issue stemming from the base of the faucet? At this point, I don't know how to proceed. Any help/insight would be appreciated, thanks.

[HankBriggs]

Since the leak is coming from the mounting hole rather than a supply line connection, and it survived a cartridge swap, this points away from the cartridge and toward a seal problem at the base of the faucet — the cartridge controls flow through the spout, but it won't fix a leak where the faucet body meets the sink deck.

Two spots I'd check, in this order:

• The base gasket/O-ring — most single-hole Moen faucets have a rubber gasket or O-ring seated between the bottom of the faucet body and the countertop/mounting deck. If that's cracked, warped, or just didn't reseat cleanly when you had things apart for the cartridge, water gets past it and finds its way out the mounting hole rather than showing up anywhere obvious. Pull the faucet, inspect that gasket, and replace it if it looks at all compressed or damaged — a light coat of plumber's grease on the new one helps it seat properly.
• The cold-side supply connection inside the faucet body — some Moen models use a quick-connect or compression fitting up inside the base rather than a visible exterior connection. A leak there can travel down and exit through the mounting hole, making it look like the faucet itself is leaking.

Since it's a Moen, it's worth finding your exact model number (usually stamped underneath or in the installation manual) and pulling up Moen's parts diagram for it — the base O-ring is a specific, inexpensive part once you know which one you need. Given no mineral buildup and hard water not being the obvious cause, a worn/misseated base seal is the most likely culprit here.

[Bodrey]

Since the leak is coming from the mounting hole rather than a supply line connection, and it survived a cartridge swap, this points away from the cartridge and toward a seal problem at the base of the faucet — the cartridge controls flow through the spout, but it won't fix a leak where the faucet body meets the sink deck.

Two spots I'd check, in this order:

• The base gasket/O-ring — most single-hole Moen faucets have a rubber gasket or O-ring seated between the bottom of the faucet body and the countertop/mounting deck. If that's cracked, warped, or just didn't reseat cleanly when you had things apart for the cartridge, water gets past it and finds its way out the mounting hole rather than showing up anywhere obvious. Pull the faucet, inspect that gasket, and replace it if it looks at all compressed or damaged — a light coat of plumber's grease on the new one helps it seat properly.
• The cold-side supply connection inside the faucet body — some Moen models use a quick-connect or compression fitting up inside the base rather than a visible exterior connection. A leak there can travel down and exit through the mounting hole, making it look like the faucet itself is leaking.

Since it's a Moen, it's worth finding your exact model number (usually stamped underneath or in the installation manual) and pulling up Moen's parts diagram for it — the base O-ring is a specific, inexpensive part once you know which one you need. Given no mineral buildup and hard water not being the obvious cause, a worn/misseated base seal is the most likely culprit here.

HB - I contacted Moen about it, explained where the leak was coming from, and they agreed that the housing probably had a crack in it or was otherwise compromised. I couldn't locate the original receipt (the faucet was over a decade old). However, they were gracious enough to send me a brand new faucet for a modest $25. Works like a champ.