Brother mfc8440

yes there was a sensor switch, which can be broken and faulty that needs to be repair or replace.

check if the printer is set where to get/expect the paper else paper sensor is faulty which is a non-user replaceable part & you need to contact brother.

Make sure the paper rollers are clean. Sometimes lint and dirt can cause paper sensor issues. USe a can of compressed air to blow away any dirt/lint from around the rollers.

If that does not resolve the issue, try a hard reset. Unplug everything, including power and PC connection. Wait about 5 minutes and reconnect everything and see if it works. If the issue persists, it could be a faulty paper sensor, which will need replacing. (Also make sure that the correct paper type has been selected from within printer properties.)

Hope that helps!

Try to lessen the volume of the paper that is loaded on the tray. Make sure the paper type is the one set on the printer's paper type setting.

Do you get any specific error code?

Regards

That means that the problem indeed may be the paper sensor. Try to turn off the unit then turn it back on and after that you may also try to reset the unit. You may consult your manual on the resetting. Also see on the part where the paper rack meets the unit itself there are sensors there try to blow on them and see if dirt may have accumulated there.