When troubleshooting an amplifier, check to make sure the amplifier is not against a conductive surface. Ensure the correct voltage is being delivered to it, and that it is ground correctly. Determine whether or not the amplifier is receiving turn-on or music input signals or if it is giving off a signal to the speaker.
An amplifier requires a non-conductive surface for mounting. If it is touching metal, either from the vehicle or the case's mounting screws, it automatically goes into protection mode. This could be the cause of an amplifier refusing to turn on.
To check that the voltage is correct, first make sure there are no loose connections or blown fuses. If the wiring is properly installed and the fuses are working, use a voltmeter to find the voltage running from the amplifier's positive to negative terminals. With the engine running, 12 volts should be present. Without the engine running, it should be around 14 volts. If no power is present, double check the wiring, particularly at intersections involving the grounding bolt, outboard fuse holder, batter connection and distribution or fuse block.
In cases where the amplifier only works when the radio is playing, consider the turn-on signal. If using a factory radio, a lead dedicated for other electronics may not be included. To fix this, employ a switched power wire, or use a unit that gathers signals from the speaker wires in order to turn on the amplifier.