During the maintenance, it should be noted that the light-emitting element of this flashlight is a "light-emitting diode", which has the characteristics of emitting light, and also has the characteristics of unidirectional conduction and reverse breakdown of the diode. First check whether there is an open circuit in the circuit, whether the switch is in poor contact, and whether the battery +-pole is corroded. When testing with a multimeter, first determine whether there is a problem with the battery. Measure the voltage across the electrode. If the voltage is 0 or below the rated voltage, consider charging or replacing the battery. At the same time, check the charging circuit for problems. After confirming that there is no problem with the circuit and the battery, overhaul the LED.
Use a multimeter × 1K or × 100 gear (high-resistance gear is not recommended, because of its high voltage, easy to cause reverse breakdown of the light-emitting diode) to measure its positive and negative on and off, to determine whether the light-emitting diode is damaged. Most LED flashlights are connected by multiple light-emitting diodes connected in series or in series and parallel, which can be detected in sections, and the damaged diode is finally found by the elimination method, and replaced by the same type of light-emitting diode.