An under-sink reverse osmosis system has several stages that wear at different speeds. The membrane costs the most to replace, and it is also the stage most exposed to whatever gets past the stages in front of it. Keeping the pre-filters current is the most useful maintenance habit you can build for the system.
Learn the stage order before you open anything
Find the install sheet or the manufacturer's manual and write down the stage order from the cold line inward. A typical sequence is a sediment filter first, then one or more carbon stages, then the membrane, then any polishing carbon after the storage tank. Match that order against the housings you can see. Stages are often different sizes, and a cartridge put in the wrong housing either fits badly or fits and does little. If the housings have no labels, label each one before you remove a single cartridge.
Why the pre-filters matter more than they look
Sediment that reaches the membrane can clog its surface. Chlorine is a particular concern for many membrane types, which is why the carbon stage ahead of the membrane is not optional. When a carbon stage is spent, the membrane takes the full load. You will not notice this right away. What you may notice is slower production, a change in taste at the treated tap, or a membrane that makes less water than it used to.
Decide who owns the change schedule
The schedule belongs to the owner, even when a provider services the unit. Decide who keeps the calendar: you, another adult in the house, or the provider under a written service agreement. Whoever keeps it, write the install date and each stage's last change date on a label inside the cabinet, and note which cartridge brand or part number was used. Without that, the next person to open the cabinet cannot tell whether the sediment stage was changed recently or several owners ago.
Shut off, relieve pressure, change, then check
Close the feed valve to the unit. Open the treated-water faucet to drain pressure from the housings. Remove each housing, swap the cartridge, and inspect the O-rings for cracks or flattening. Lubricate them only with the grease the manual specifies, and refit each housing hand-tight plus whatever the manual calls for. Turn the feed back on slowly, run the faucet for the flushing time the manual gives, then check every housing and fitting for drips before you walk away. A cracked housing is an easy way to create a leak, so handle the plastic with care.
Signs the schedule has slipped
Watch for a drop in faucet flow, a taste or odor change at the treated tap, a cartridge that looks dark or slimy when opened, or a membrane that makes less water than it did. A handheld TDS meter can show whether dissolved solids in the treated water are rising, though it will not catch everything. When you see any of these, check the pre-filters first. They cost less to replace than the membrane, and a clogged stage can make a healthy membrane look failed.
Keep a one-page checklist on the cabinet door
Include the stage order, the part number for each cartridge, the change interval from the manual, the last change for each stage, the shutoff valve location, and the phone number of whoever services the unit. A checklist that lives where the work happens gets used. One stored in a drawer does not.