Toner gets in the developing assembly toner sensor (TS1) and may be shorting out the sensor. Blow out any toner in the sensor wiring and check to see if measure resolved the symptom. The toner level sensor has an open-circuit, replace the toner sensor.
Toner Sensor FK2-0358
However, re-attachment of the sensor also solves the symptom.
When the symptom has occurred, make the following selections in service mode [COPIER > I/O > DC-CON] and check the setting for the bit1 of [P021]. If the setting is "1", re-attach the toner sensor.
One problem found has been the toner sensor scraper blade in the developer not making contact with the surface of the toner sensor.
To check the toner sensor scraper blade first set up a work area. Lay out several sheets of 11 x17 paper on the work surface. Disassembling the developer is a messy job. Remove the upper cover of the developer and then rotate the developer in a counter clock wise direction. Basically the cylinder should rotate so the toner is scraped off of the cylinder by the developer blade.
If it is rotated in the wrong direction a large amount of toner will adhere to the developer cylinder and makes a mess.
The toner sensor is located on the on the outside of the developer and part of it goes through the wall of the developer unit.
Next look at the inside surface of the developer where the toner sensor is mounted while rotating the developer cylinder in the correct direction. There is a mylar scraper sheet that needs to touch the inside surface of the developer wall. In the image below the inside surface of the developer wall is being touched by the mylar scraper blade. If the blade is damaged or broken off the toner will not be cleaned off of the sensor and give the error code E024. If the blade is the problem the developer will need to be replaced.
The part number for the developer is FM2-0287-010.