Check the fuse or breaker box before doing anything else if the compressor will not run. It is possible that the compressor is drawing a lot more power than the circuit can provide, thus tripping the breaker or burning the fuse out. If the compressor is connected to an extension cord, it must be connected to the right type of extension cord in order to be safe. For example, if the extension cord is 150 feet long, it should be a 10-gauge (AWG, or American Wire Gauge) cord so it can handle the voltage. If this is not the issue, the pressure switch may need to be replaced. New parts can be ordered from Campbell Hausfeld. Also, it could simply be that the tank is fully pressurized and does not need to charge. Using the air in the tank will cause the pump to engage and run.
Check the pump motor with a voltmeter if the motor hums but either will not run or runs slowly. The motor may also be connected to an extension cord that is too small. Attaching the air compressor to a 10-gauge extension cord should allow enough voltage to pass to the motor. If this isn’t the problem, the motor may have a short in the windings or a defective check valve or unloader. Both of those issues require the attention and expertise of a qualified service dealer.
Unplug the air compressor and reduce the air pressure inside the tank if a lot of moisture blows out of tools with discharge air. This indicates that there is an excessive amount of water inside the tank and it must be drained. After releasing the pressure in the tank, place a catch pan under the unit and open the tank drain underneath the tank. The water in the tank will drain out readily. This will occur over and over again unless you do one of the following: use the air compressor in an area with less humidity, or use an air line filter.
Drain the tank if the motor starts and stops excessively. Water takes up space that could be otherwise filled with air, so by draining the tank, you will free up extra space in the tank to be used for air. Use an air-line filter to help keep air out of the tank. It is also possible that air is leaking out of connections in the hose or piping. Use a soap and water solution to test each joint for leaks and tighten or replace the joints that leak. The check valve may be leaking, too. Perform the soap and water test on it, too, to check for leaks. If it leaks, remove the check valve, wrap it with Teflon tape, reinstall it, and test it again for leaking.