Service Information for the  Lexmark T644, T642, T640 Error Code 920.04
This is one of those errors that is oddly specific so I’m not exactly sure of when it is actually happening. But the main point is that the hot roller is cooler than what it should be after an expected temperature change. Below you will find some service information on the  Lexmark T644, T642, T640 Error Code 920.04 along with links to purchase a replacement fuser if needed. Also, at the bottom of the page you will find a link to the fuser replacement instructions.
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Premium Quality Compatible Fuser Kit – Lexmark 40X2592
Lexmark T644, T642, T640 Error Code 920.04
Hot roll too cool when heating to desired temperature after slope change (standby control only).
Fuser page count  between 0 and 99,999.
Service Manual Service Information for the Lexmark T644, T642, T640 Error Code 920.04
• Bad thermistor, circuit or cabling.
• Fuser not receiving sufficient AC  power.
• Poor supply line voltage.
• Low wattage or incorrect lamp.
• Incorrect hot roll.
Additional Service Information for the Error Code 920.04
I’m not going to get into lots of details on this one. The service manual tips are pretty good. Turn the machine off then back on and see if it repeats. If it happens again in the next month or so then further investigation is needed. Make sure the machine has its own power source and is plugged straight into a wall outlet for testing purposes. Remove the fuser and check the contact points on the lamp and make sure the thermistor is clean of debris and is making contact with the surface of the hot roller. If you still have fuser errors then replacement of the fusing assembly is the next step in the troubleshooting process.
Click on the link below for the fuser replacement instructions.
Lexmark Optra T640, T642, T644 Fuser Removal Instructions
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