Not every window fault means a new window. This section starts from the symptom you can actually see - a draft, fog between the panes, water, rot, a sash that will not move - and works back to what is likely failing, so you know whether you are buying a repair or a replacement before anyone quotes you.
The order matters here. Diagnose first, then decide the scope, then get quotes. Done the other way round, you get quoted for whatever the person in your house sells.
Start from the symptom
Fog or haze between the panes. The glass is sealed as a unit, and this is what it looks like when that seal has failed. The frame around it may be entirely sound, which is why this symptom in particular is worth diagnosing before agreeing to replace a whole window.
A draft you can feel. Air is moving through something. That something may be the sash, the weatherstripping, the frame, or the gap between the frame and the wall behind the trim - and those are four different repairs at four different prices.
Water on the inside. Worth separating from condensation. Water tracking in during rain points at flashing or the seal to the wall; water appearing on cold mornings is usually condensation, which is about humidity and surface temperature rather than a leak.
Rot, soft wood, or flaking. Where it is - sash, frame, sill, or the structure behind - determines whether this is a window job or a bigger one.
A sash that sticks, drops or will not stay up. Often hardware, balances or paint rather than the window as a whole.
Repair or replace
This is a genuine decision, not a formality, and the honest answer is that it depends on what is failing and on how much life is left in the rest of the unit. A failed sealed unit and a rotten frame are not the same finding.
Where a page below carries a figure for either path, it is cited to a named source with the date it was reviewed. The general cost picture is in What It Costs.
A note on the size of this section
This section is smaller than the others on this site, and deliberately so rather than by oversight. The keyword research behind this site found very little search demand on window fault diagnosis compared with replacement and pricing, so we have not manufactured pages to fill it out. It will grow if and when there is something sourced to say.
Common questions
Does condensation on my windows mean they have failed?
It depends where the condensation is. On the room-facing surface it is about humidity and surface temperature. Between the panes, where you cannot wipe it off, is a different matter and is covered above.
Can you diagnose my window from a description?
No. This site is not a contractor, does not inspect, and cannot tell you what is wrong with a specific window. What it can do is narrow down what to have someone look at, so the visit is a shorter one.
Is a repair always cheaper than a replacement?
Not always, and not always the better value even when it is cheaper, because a repair on a unit near the end of its life buys less time. That trade-off is the actual decision here.