How do you print duplex on a HP Laser Jet 4650n

Are you sure duplex option in this model I wont think so, pl verify the product specifications..

Do you have the 1320D or just a 1320. Only the D version will duplex automatically. The base version requires you to manually duplex. If you are sure you have the duplex version, then check the settings in the print driver and make sure it shows that the duplex option is installed. The driver often defaults to no duplexer.

Click Start
Click Control Panel
Click Printers
Select the HP printer
Configure it to "double-sided" or "duplex" mode.

Printer error code "49.4c02" is one of them. It occurs when there is corrupted data. This may confuse the printer, and it stops functioning. In other cases, the data is corrupted while being transferred to the printer. Some printer models are naturally prone to this error because of their driver, while others can display this error while printing a specific font.fix printer error 49.4c02
Hope this helps you.

check your software, and printer settings

To prevent this you would have to set printer for single sided printing. This is an automatic feature in hp printers mine does the same thing.

according to specs

http://www.shopping.hp.com/shopping/data_sheet/q3669a.html

this supports MANUAL duplexing only.

HI ayyasamyr:
This your problem.
13.10 PAPER JAM The event log message for a paper-delay jam in the duplexer is 13.10.
1. Check the duplexer and the rear area of the printer for obstructions or damage.
2. Check PS1701 and PS1703 in the duplexer for correct operation. Replace the
duplexer if a sensor is defective.

Hope this helps

I believe you are being referred to the "Users Manual"
Do you still have the book that came with it?

Duplex on this printer is manual, that means that you will have to re-insert the paper by yourself.

This if you have the 2420n, the 2420dn instead is the duplex model.

Here specifications, look where it says duplex, 2420n had only manual duplex (the printer wait for you to re-insert page).