The answer to 'why is my pressure washer mildew' is that mildew can build up in the pressure washer's water supply lines, pump, and other internal components over time. This is often caused by leaving stagnant water in the system after use, which can lead to the growth of mildew and bacteria. Symptoms of mildew in a pressure washer include a musty odor, visible mold or fungus, and poor performance.
To address this issue, it's important to properly maintain and clean your pressure washer after each use. This includes draining any remaining water, flushing the system with a cleaning solution, and allowing it to fully dry before storage. If the mildew problem persists or you're unable to fully clean the system, it's best to call a professional pressure washer repair technician to inspect and service the equipment.