The internal switches of a controller, called MOSFETs or power transistors, switch current on and off to control solar current and load current. Because these components are in the power path, the controller will no longer be able to switch current if they fail.
Power transistors can fail for the following reasons:
Over-voltage: lightning or other high voltage transients on the PV power cables.
Over-current: too much PV for the controller or a short in the PV line pulls too much currentout of the battery through the controller.
Extreme temperatures during charging.
A premature failure of the component. The transistors are solid state devices that are manufactured in huge volumes with extremely low failure rates. However, no electrical components are 100% defect free and can pass test and then fail after a short period of time.
Old age