A bypass valve lets water route around the treatment equipment, so you can isolate a filter or softener for service without shutting off the whole house. That is useful, but it also means the equipment can sit in the house looking installed while untreated water flows straight past it. The failure is quiet. Nothing alarms, nothing drips, and the system still looks fine from across the room.
Know what the valve does in your setup
Most installations use a set of valves at the inlet and outlet of the treatment equipment, often a three-handle arrangement or a single lever on a manifold. One position sends water through the equipment, and another sends it around. Some valves are labeled with words such as service, bypass, treat or on, and some are marked only with arrows. The labeling varies by manufacturer and by installer, so the marking on your own valve, and the installer's note if you have one, is the only reliable guide.
Take a photo of the valve set with the system running correctly, and write down what each handle position means. Keep the note near the equipment. Anyone who touches the valves later, including you, will then have a reference.
Find the moments it goes wrong
A bypass is most often left in the wrong position right after work on the system. A cartridge or media change, a service visit, a repair to a leak or a plumber working on a nearby fixture are the usual points. The person doing the work may open the bypass to isolate the equipment, complete the job, and then forget to return the handles. Some valves also look nearly identical in both positions, so a quick glance can miss the difference.
After any service, check the valve set before you leave, or within an hour of the visit, and confirm the handle matches the photo you took.
Recognize the early signs
The first clues are usually in the water rather than in the equipment. Watch for these changes over the next few days:
Hard-water marks returning on faucets, shower glass or dishes faster than they used to, especially after a period when the water seemed clean. Soap that lathers poorly or leaves a film. A taste or smell in the water that you associated with the unfiltered supply before the equipment was installed. A pressure gauge on the treated line reading the same as the untreated line, if your installation includes gauges on both sides.
None of these proves a bypass problem by itself, since a failing cartridge or a softener running out of salt can cause similar changes. But if the change started right after a service visit, the bypass is the first thing to check.
What to do without forcing anything
Compare the valve set to your photo. If the handles are in the wrong position, return them to the treat position slowly, one at a time, and watch the water at the nearest tap for a few minutes. If a handle is stiff, sticking or leaking, stop. Do not force it. A valve that will not move cleanly needs a plumber or the installer to look at it, and a leak at the valve body is not a homeowner task.
If you cannot tell which position is correct, shut the water supply to the equipment as your installer described, and call the installer before you run the system further. Running the house on untreated water while you guess is the worst outcome, and the photo and written note you made earlier are the fastest way to end the guessing.