My printer the gear the drum its crash

Try Engine test:
turn on device open tray 1 and load paper (1 is the manual feeder)
when you open tray 1 there's an engine-test switch, found it and press it

Engine error are also cause too much resistance from toner, fusor or gears. Remove toner and check that it roll with your fingers on gears, and do same with fusor (could be very hot warning - should be hard move it but not too much) and with printer off and fusor removed check if you can move any gear manually (this can be harder but will help you determine if it's board error or resistance cause a gear problem).
Any previous error happened with the printer can help to diagnose (jams, power off while printing...)

front, rear ? I'd say fuser assembly gears or drum on toner cartridge; check the gear teeth on the toner cartridge's drum.

It sounds like the teflon film is coming off the fusor roller (which would explain the jamming).
I suggest replacing the fusor assembly on one of the units, and if the problem is resolved, then do so for the rest of the models.
If it's hapening on a great number of units, you may want to call HP at 1-800-HP-Invent and see if it's a known problem that they are covering for free.

This machine has a separate drum and toner cartridge. Replacing the toner does not reset the drum light. Double check that the light on is, in fact, the drum light, and if it is, replace the drum unit. This should resolve your problem.
In order to replace the drum unit, do this: 1) With the printer on and the drum light blinking, open the front cover. 2) Press and hold the Go button for about 4 seconds until all the LEDs light up. Once all LEDs light up, release the Go button. 3) Pull the print assembly out of the machine. 4) Push down the green lock lever to take the toner cartridge out of the drum unit. 5) Put the toner cartridge into the new drum unit. 6) Reinsert the print assembly into the machine. 7) Close the front cover and verify that the lights turn off.
It is theoretically possible that you could reset the counter without actually changing the drum by simply skipping steps 3-6 above, but I have never tried it before and Brother may be checking a chip on the drum unit to verify that it is, in fact, a new drum unit. If they are, you will have to actually replace the unit to make the light go off. I would strongly recommend against resetting the counter without actually changing the drum though, as your print quality will degrade and the machine may start to dump toner into the inside creating a rather large mess and a hefty repair bill.

This is a problem with the fusor not heating in the middle for some reason. Either the fusor roller is damaged or the heating element is.
Toner is not like ink, it has to be heated onto the paper.
Take a good look at the roller...if it's not damaged, then the fusor itself has a problem and the unit in either case must be serviced.

Sounds like a broken drum drive gear. Try another known good drum assembly first. Parts can be googled and found. But, difficult for end user to replace. You have to remove all covers and fuser asm. Then gear housing from the left side to change the drum drive gear.

I’m happy to help further over the phone at https://www.6ya.com/expert/jayw_0e86fb055a7b73de

That could be a problem with either the fusor wire or just need to reset the darkness setting or you didn't gently shake the cartridge (or the toner was exposed to high humidity).

The fusor wire may not be getting hot enough to get all of the toner to stick to the paper and you get the faint page. The fusor assembly is wirth about $240 so you want to make sure if it might be covered by warranty and check the replacement price for the printer. The replacement price might be close to that and you would end up with a new printer with warranty.

check the pressure of the fuser adjustment

The fuser is most likely your candidate. A drum transfers the image onto the paper, but the transfer is made with an electric charge. The fuser applies heat and pressure to bond the toner to the paper.

If the fuser rollers are worn or there's heavy buildup on them, they will cause the paper to skew, crease and/or jam.

In the case of the paper bunching up and looking like an 'accordion', if this happens at the entrance guide to the fuser assembly, you're pretty much guaranteed the fuser's either jammed or defective.

I have 'reused' the same drum three times and still prints like a charm.
Open the toner door and pull out the toner/drum assembly. Take the toner off the drum tray. Use a alcohol pad or lint free cloth and wipe the mauve coloured drum to remove the excess lint and toner collected on the drum. Rotate the drum with the white gear and keep cleaning until the whole drum is done.
Put the toner back in the drum tray. Clean the corona wire by sliding the blue tab back and forth 3 or 4 times. Put the toner/drum back into the printer and close the door.
After the printer does the self test the drum light will cone on again so then you need to reset the counters.
Open the door again
Hold the GO button until all 4 lights are lit up
Release the GO button
Close the door
Printer will self test and should extinguish all the trouble lights.