A reverse osmosis system is a small number of parts and most of the trouble it causes is a part you do not have when you need it. Keeping a short shelf of consumables turns most faults into a twenty minute job, and the list is cheap. It also makes filter changes more likely to happen on time, which is where the system's performance actually comes from.
The consumables
A set of prefilters, meaning a sediment cartridge and a carbon cartridge, is the core of the list, and having them on the shelf is the single best predictor of whether changes happen on schedule. A spare membrane is optional, since it is replaced rarely and on evidence rather than on a calendar. A polishing carbon filter goes with the prefilters. If the system has a remineralization stage, that cartridge belongs in the same set. Buy them as a kit for the specific system rather than individually.
Seals and small hardware
Housing o-rings are the part that turns a filter change into a leak, and they cost almost nothing. Keep at least one set, along with a tube of food grade silicone grease. Keep the sump wrench that came with the system somewhere you will find it, since improvising with a strap wrench on a tight housing is how housings get cracked. A few spare push fit collets or inserts and a small tubing cutter cover most of the connection problems that arise.
Tubing and the parts that fail quietly
A length of the correct diameter tubing is worth having, because tubing gets kinked when the cabinet is rearranged and a kinked line looks fine while throttling production. The check valve and the automatic shut off valve are the two components that fail without an obvious symptom, producing either a system that runs continuously to drain or one that stops filling, and both are inexpensive. The tank's air valve is worth knowing the location of, since tank pressure is the most misdiagnosed fault in the whole system.
Knowing what is actually wrong
A short diagnostic order saves replacing the wrong thing. Slow production: check the prefilters, the line pressure and the tank air charge before suspecting the membrane. Continuous drain flow: check the shut off valve and the check valve. Poor taste with normal flow: check the polishing filter and the membrane's rejection with a dissolved solids reading taken before the tank. Leaks after a change: the o-ring, almost always. Our RO installation guide covers the layout these parts sit in.
Questions people ask about ro system parts
What should I always have in stock?
A prefilter set, spare housing o-rings, silicone grease and the sump wrench. Those cover routine changes and most leaks.
Should I keep a spare membrane?
Optional. It is replaced rarely and on evidence rather than on schedule, so most households order one when a reading shows it is needed.
Which parts fail without an obvious sign?
The check valve and the automatic shut off valve, which cause continuous drain flow or a tank that will not fill, and the tank's air charge.
The system is slow. What do I check first?
Prefilters, incoming line pressure and the storage tank's air charge, in that order, before assuming the membrane is finished.