A carbon filter does not stop working all at once. As the carbon fills up with the material it has been capturing, some contaminants begin to pass through again. The first sign is often taste or odor, and it usually shows up before anyone thinks to check the filter. This is sometimes called breakthrough, and recognizing it early is the owner's job.
The early signs to watch
The most common sign is a chlorine-like taste or smell that was absent after the last change and now shows up at the kitchen tap, in drinking water, or in coffee and tea. Some households notice a change in the water's odor when it has been sitting in a glass overnight. Note the first day you notice it, and whether it is worse at certain times of the day or after heavy use.
Check whether the change interval was set by time or by volume
Carbon filters are rated for a certain amount of water, and many are also changed on a calendar. Ask your provider which interval the filter was set to, and whether that interval assumed your daily water use. If the household uses far more water than the provider estimated, the filter can run out early even when the calendar says it should last longer.
Rule out other causes
Taste can also come from a water heater, a dirty faucet aerator, a bacterial problem in the supply, or a change in the source water itself. Run the cold tap for a minute before you judge the taste, and compare the filtered tap with an unfiltered one if your system allows it. If the change happened right after a service visit, ask the technician whether the filter was installed correctly.
Do not wait for the calendar to finish
Spent carbon can hold material that it is no longer removing, and it can support bacterial growth. For that reason, a taste or odor change is a reason to schedule a change, not a reason to hope the filter lasts until the next date on the schedule. Ask your provider how quickly a change can be made and what they will check on the visit.
What to write down and ask
Keep a short record: the date of the last change, the date you first noticed the taste, what you observed, and any water use changes such as guests or new appliances. Ask the provider what media the filter uses, how the change interval was calculated, and what the warranty or service terms say about early replacement. For a broader approach to comparing providers, see Methodology: How We Match Systems and Compare Providers.