The answer to 'why is my pressure washer mildew' is that mildew can build up in the pressure washer's pump, hose, and other components over time. This is often caused by leaving water sitting in the system after use, which can lead to the growth of mildew and other microorganisms. Mildew can clog the system and reduce the pressure washer's performance.
To fix mildew issues, you'll need to thoroughly clean and maintain your pressure washer. This includes flushing the system with a cleaning solution, replacing any damaged parts, and properly storing the pressure washer when not in use. If the mildew problem persists or you're unsure of the cause, it's best to consult a professional pressure washer repair technician.