Dirty contacts on Upper Right Cover Safety Switches. Toner gets inside the Upper Right Cover Safety Switches, which can lead to lower DC voltage being applied to the Laser Diode. Which leads to:SC 990 eng_procon.c (when a normal Grayscale pattern was not detected 15 times consecutively during ID Sensor Grayscale pattern check) check with SP 7403 Service Code History look for Detail in history of SC 990.LD Power not being strong enough to be read by Laser Synchronizing Detector causing a SC 322 or 326 depending upon timing.5vdc not being fully applied to connector 403 pin 3 on the Laser Diode PCB causing an Open Cover Indication, or 24vdc not being applied fully to connector 107 pin 2 on the PSU PCB causing an Open Cover Indication.Circuit Explanation: 5vdc (VCCD)is output from connector 403 pin1 of the Laser Diode PCB where it travels through 2 of the Upper Right Cover switches than through 2 of the Front Cover Safety switches before being applied to connector 403 pin 3 of the Laser Diode PCB where it is now used to power the Laser Diode. Lower voltage on pin 3 leads to lower laser diode output which causes the machine to raise LD Power (SP 2-103) when maximum values are reached, laser diode output can not be increased further which can lead to SC 322, SC 326 and/ or SC 990 eng_procon .c. If the voltage drops too low an open door indication is sent to the BICU PCB but a signal is not sent to the IOB PCB which tells the machine which door is open.Note: An older Drum more than 150,000 copies can also lead to SC 990 by causing incorrect ID Sensor readings[Resolution]1. Clean the internal contacts or replace Right Door Safety Switches (2 of Part Number 1204 2521 and 1 of Part Number 1204 2529)Clean and vacuum the PCU.Clean the Toner Shield Glass, Charge Roller and ID Sensor.Check Service history for Drum Life, if over 150,000 replace Drum (Part Number A230 9510)Reset SP 2103 -1 thru 4 to defaults.(LD Power Adjustments)Reset SP 2001-1 to its default. (Charge Roller Bias Adjustment)Execute SP 3001- 2 (ID Sensor Initial Setting)