Almost everything in the tank is a repair
The working parts of a toilet are a handful of inexpensive components, and replacing any of them is cheaper than replacing the fixture by an order of magnitude.
The flapper — the rubber seal at the bottom. Hardens with age and its seat scales up on our water, which is what causes a toilet to run. Food colouring in the tank with no flushing proves it within fifteen minutes.
The fill valve — refills the tank and shuts off. Symptoms are refilling too slowly, not shutting off, or a whistling noise.
The flush valve and its seat, if the flapper keeps failing on an eroded surface.
The handle and lift chain, which is the cheapest fix of all and causes both weak flushes (too much slack) and running (too little).
A full rebuild kit replaces essentially every moving part and still costs a fraction of a new toilet. On a fixture you otherwise like, that is nearly always the right answer.
The three that mean replacement
Cracked porcelain. Not repairable, whatever the internet says about epoxy. A hairline crack in a tank or bowl is under water pressure and thermal cycling, and it fails eventually — sometimes all at once, with the tank's full volume on the floor. A crack below the waterline means replace it now.
Scaled or crazed bowl interior. On hard water, mineral deposits and a worn glaze eventually make a bowl impossible to keep clean. If it stains again within days of a thorough clean, the surface is gone rather than dirty.
A pre-1994 water hog. Toilets from before the 1.6-gallon standard use three to five gallons per flush. On the most-used fixture in the house that is the single largest water saving available, and it usually pays for itself.
Worth adding a fourth, softer case: a toilet needing its third or fourth repair in a couple of years. Individually each is cheap; collectively you are maintaining a fixture that is telling you something.
Worth knowing
A rocking toilet is not on this list, and it is important that it is not. Rocking is a flange or floor problem, not a fixture problem — and tightening the bolts against a corroded flange cracks the porcelain, turning a seal repair into a replacement. That is one of the most common ways people convert a small job into a large one.
Choosing a replacement
Four decisions, and one of them is the only measurement you must get right.
Rough-in. The distance from the finished wall to the centre of the closet bolts. Twelve inches is standard; ten and fourteen exist in older homes. Measure before buying anything — the wrong rough-in does not fit, and nothing else about the toilet matters if it does not.
Bowl shape. Elongated is more comfortable; round saves a couple of inches in a tight bathroom.
Height. Comfort height (around 17-19 inches to the seat) is easier for most adults to use and noticeably easier for anyone with mobility or knee problems. Standard height suits children better.
One piece or two. One-piece units have no tank-to-bowl seam to leak and are easier to clean. Two-piece are cheaper and lighter to handle.
Ignore the marketing about flushing power and look for MaP testing figures, which are measured rather than claimed. A low-flow toilet that has to be flushed twice has saved nothing.
What a proper installation involves
The parts people skip are the parts that cause the callbacks.
The flange gets inspected, not just re-used. It is the fitting anchoring the toilet to the drain, and on older homes it is frequently corroded, cracked, or sitting below the finished floor after a flooring change. A flange that is too low cannot compress the seal properly, which is why a new toilet can leak from day one. Repair rings and extenders exist for exactly this.
A new wax ring or gasket every time. They compress once. Re-using one guarantees a leak.
The closet bolts get replaced with brass, which does not corrode.
Tightened alternately and gently until snug, never further. Porcelain does not flex, and overtightening is how new toilets get cracked during fitting.
The supply line is replaced, not re-used, and the shutoff valve gets checked while everything is accessible — that is the cheapest moment to replace a stiff or weeping one.
Then it gets tested: flushed several times, and the base checked dry after each.
The parts of this worth doing yourself
Tank internals are genuinely straightforward. A flapper takes ten minutes with no tools, and a full rebuild kit is an hour and comes with instructions. If a toilet runs or flushes weakly, that is a reasonable place to start.
Full replacement is more involved than it looks — a toilet is heavy and awkward, the flange work is where the judgement lives, and porcelain is unforgiving of being over-tightened or dropped.
Call if the flange is damaged or below floor level, if there has been water around the base for a while (the subfloor needs checking, not just the seal), if the shutoff valve will not close, or if the floor feels soft anywhere near the toilet.
That last one matters most. Water at the base means every flush has been putting waste water under the floor, and the seal is the cheap part of that repair.