If you are talking about the Macerator toilet and the flush panel showing green and RED. The Black tank has a dedicated sensor just for the Toilet and IF that sensor is showing the tank over 3/4 full it will stop the toilet from flushing(red light). So, if the regular Black tank level monitor shows below 3/4 full and the red light comes on either the Toilet sensor is dirty or the sensor is defective. It is critical to flush a black tank completely when dumping when you have this type toilet. If the tank is at 3/4 or higher then the system is operating correctly. You need to dump the tank. Doug