Over time, Lithium-Ion cells degrade. If one cell in the battery pack fails (goes to 0 volts), the Battery Management System will send an "invalid state" signal to the vacuum handle. On many Philips models, a faulty battery pack manifests as an E4 error rather than a battery icon.
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: Turn off the vacuum immediately to prevent motor strain. philips speedpro max e4 error
: If the error occurs on thick rugs, the resistance may be too high. Try switching the vacuum off and back on again on a different surface. Over time, Lithium-Ion cells degrade