A hot spot is a cell, or a small region of a cell, forced to dissipate power as heat while the rest of the string keeps pushing current through it. The cell has become a load. Shade, a crack, a bad solder bond, or a bypass path that did not conduct can put it there. This page will not name a temperature at which that heat becomes dangerous. The dangerous condition is the mechanism, and the temperature a given module reaches is a measurement, not a slogan.
How a cell is forced to dissipate
Cells in a module are in series inside their group. The current is common. A cell that can no longer pass that current at the string's operating point is driven into reverse by the cells around it, and it burns the difference as heat. A soft shadow can be enough. So can a crack that disconnects part of the cell, so that a smaller area tries to carry the group current. How ordinary shade reduces output, before anyone calls the result a hot spot, is in how shade affects PV output.
The bypass diode exists to give that current another road, around the weak group, so the weak cells are not the road. When the diode conducts, the group is skipped and the heat in the shaded cells falls. When the diode has failed open, the road is gone and the cells take the current. That failure mode, and the opposite case of a diode that stays shorted, is in when a bypass diode fails. A hot spot and a diode fault are neighbors. They are not the same sentence, and replacing a diode is not how a homeowner treats a warm module.
Soiling in a hard edge, a leaf that sits for weeks, a bird dropping, or a railing shadow that parks on one cell at noon are ordinary triggers. A module that was fine in a lab flash test can still hot-spot on a roof once the shadow is local and persistent. Moving the shadow is the first useful act, when the shadow is a weed or a loose object you can reach without climbing. A shadow from a vent pipe or a neighboring story is a design fact. It will return every sunny day at the same hour.
What you can see without opening anything
From the ground, with the array connected as usual, an owner can look for a persistent stain, a shattered cell, browning of the backsheet in one spot, or a blister. Those are clues, not a diagnosis. A module can run a hot cell and still look clean. Production that falls on one string while the neighboring string, at the same tilt, holds up is another clue, and the inverter or the string monitor has to be read by someone who knows which string is which. Do not unplug connectors to compare strings. Opening a connector under load is live DC work.
Infrared cameras and IV tracing are how a qualified person confirms a hot spot, and they do it under conditions they choose, with the training to tell a warm junction box from a hot cell. A phone thermal image taken through glass, at a steep angle, in the wind, is a picture. It is weak evidence. Handing that picture to a technician is reasonable. Declaring a cell dead from it is not a measurement this article will endorse.
Who may test, and what stays closed
A qualified person tests the module and decides whether it stays, whether a string is reconfigured, or whether the module is replaced under the warranty process the manufacturer actually offers. They keep the junction box closed unless their procedure and the manufacturer say otherwise. The diodes and the terminals in that box are part of a listed assembly. Opening it to poke a diode exposes live parts on a sunlit module, and it can end the warranty before anyone has learned what failed.
Until that person arrives, remove only the shade you can remove from a safe place, leave the connectors mated, and do not tape over the glass or spray water on a hot module. Note the time of day the shadow falls, and which module, counted from a corner you can describe. That note is the useful work. A hot spot that is really a failed diode or a cracked cell will still be there after the leaf is gone, and the note tells the technician whether they are hunting a shadow or a defect.
