A bypass diode fails in one of two electrical ways that an owner can understand without opening anything. It fails open, so it cannot conduct when a group of cells needs a way around a shadow or a crack. Or it fails short, so it conducts all the time and that group stays out of the circuit on clear days as well as cloudy ones. The string behaves differently in each case. Neither repair is a homeowner task.
Open diode: the weak group stays in the path
In normal sun the diode sits reverse-biased and carries essentially no current. The cells work. When a group is shaded or cracked, the diode is supposed to turn on and pass the string current around that group. An open diode cannot turn on. The string current is forced through the weak cells, which then dissipate heat. That is the hot-spot mechanism described in what a hot spot is on a solar panel. The module may look only a little weaker in full sun, because the diode was idle then, and it may fall apart electrically as soon as a small shadow crosses that group.
The rest of the string still tries to push its current. The inverter's tracker may find a lower-power point, or the string may show a shape a technician recognizes on an IV trace. Those traces are the technician's tool. Unplugging the module to see if the string recovers is live-DC work on a sunlit connector, and it is not a test this page will walk anyone through.
Shorted diode: that group is parked out
A shorted diode holds its group in bypass continuously. The cells in that group stop contributing voltage, in sun and in shade. String voltage sits lower by whatever that group normally added, and the rest of the string can still produce. You have lost a slice of the module for good, until the module is repaired or replaced by the procedure the manufacturer accepts. Heat in the shaded cells is less the story, because current is not being forced through them. Heat in the diode itself can become the story, because the diode now carries string current whenever the string is working. What a string is, and how one module's voltage sits inside it, is in what a solar string is.
A string that runs cooler in voltage than its twin, by about the span of one diode group, is a clue a qualified person can measure. This page will not print that span as a number of volts. It depends on how many cells the diode guards, which the module drawing shows, and on the temperature at the time of the measurement. Comparing two strings with a clamp meter and a guess is how the wrong module gets condemned. The drawing plus a proper measurement is the diagnosis.
Leave the junction box closed
Bypass diodes live in the junction box, often potted, on a module that is a listed assembly. Opening the box, lifting a diode, and soldering a replacement from a parts drawer changes that assembly. It exposes terminals that are live in daylight. It can void the warranty. It can leave a joint that fails the next time the roof heats up. The person who may do anything inside that box is a qualified person following the manufacturer's procedure, which often means replacing the module rather than repairing the diode in place.
From the ground, the useful record is which string is down, at what time of day, and whether a shadow sits on one module when the loss appears. An open-diode problem often shows up with partial shade and quiets, electrically, in uniform sun. A shorted diode is present in uniform sun as a permanent loss of that group. Telling those patterns apart narrows the visit. It does not authorize a ladder and a screwdriver. Half-cut modules and full-cell modules both use diodes. The failure modes above apply to the group that diode protects, whatever the cell size.
