Carbon cartridges fail in four recognisable ways and three of them are not faults at all, just the media doing what it does at the end of its life. Knowing which symptom means what saves both unnecessary replacements and, more expensively, running a cartridge long past the point where it stopped helping.
The taste has come back
This is the normal end of life and it usually arrives gradually enough that a household adapts without noticing. Carbon has a capacity rather than a visible end point, and once its surface is loaded, chlorine and taste compounds pass straight through. Nothing about the cartridge looks different. If the taste has returned and the cartridge is near its rated volume, replace it. If it has returned early, look at whether the water has changed: a utility switching to a chloramine burn will defeat a standard carbon cartridge that was coping with free chlorine.
The water ran black
Fine carbon dust from a new cartridge, which is normal and clears with the flush the manual specifies, usually a few litres. If black water appears from a cartridge that has been in service for months, that is different: the media is breaking down, which happens when carbon has been degraded by chlorine over time or when the element has been run far past its capacity. Replace it, and check whether a sediment prefilter upstream has been missing or blocked, since that accelerates the breakdown.
Pressure has dropped
Carbon adsorbs rather than strains, so a carbon element should not block from doing its job. Falling pressure across a carbon cartridge means it is catching particles it was never meant to handle, which points at a missing or exhausted sediment stage upstream. Fit or replace that, because a carbon element being used as a sediment filter is an expensive way to remove grit and its actual capacity is being wasted on particles rather than spent on chlorine and taste.
A musty or stale smell
This is the one worth acting on quickly. A warm, wet cartridge held past capacity is a comfortable place for bacteria to grow, and the smell is the result. It is most common after a system has stood unused, or in a cartridge that is a year over its interval. Replace the element, sanitize the housing if the manufacturer provides a method, and flush thoroughly. Then set the interval from rated volume and write the date on the housing. Our replacement cartridge guide covers matching elements to housings.
Questions people ask about carbon water filter cartridge
Why has the chlorine taste come back?
The carbon's surface is loaded. It has a capacity rather than a visible end point, so nothing about the cartridge looks different.
Why is the water black?
Carbon fines. Normal from a new cartridge and cleared by flushing; from an old one it means the media is breaking down.
Why has my pressure dropped?
The carbon element is catching particles, which means the sediment stage upstream is missing, blocked or exhausted.
What causes a musty smell from the filter?
Bacterial growth in a cartridge held past capacity, often after a period of disuse. Replace it, sanitize the housing and flush.