DIAGNOSIS Sensor calibration is off, or toothpaste foam is interfering with the brush head's movement, making the handle think it's under constant pressure.
FIX Perform a sensor reset. With the brush off, press and hold the power button for 20 seconds until all lights cycle and it vibrates. This reboots the system. Try brushing with less paste and see if the red light stops.
IF THAT DIDN'T WORK The issue might be the specific brush head. Try a different, new iO brush head. A worn-out or damaged head can move irregularly and trigger false pressure warnings.
HEADS UP The pressure sensor is very sensitive to lateral pressure (pushing sideways on the teeth). Focus on letting the micro-vibrations do the work, not scrubbing like a manual brush.