Question : Problem: HP Laserjet 4100 Getting Cannot Duplex  Error

Have tried other duplexers, still getting cannot duplex error. HP Laserjet 4100

Answer : Problem: HP Laserjet 4100 Getting Cannot Duplex  Error

I assume this printer is out of warranty. If it isn't, I'd get HP to look at it.

If not, see if you can get the printer to duplex itself, by performing a paper path test, as follows:

To perform a Paper Path Test:

Press MENU until Information Menu appears.
Press ITEM until Print Paper Path Test appears.
Press SELECT.
Press + to cycle selections until the desired paper tray appears.
Press SELECT.
Press + until the desired output bin appears. (Open the Rear Output Bin to print to it.)
Press SELECT.
Press + until the desired duplex mode (on or off) appears.
Press SELECT.
Press + to cycle selections until the desired number of copies appears. Choose 1, 10, 50, 100, or 500 copies.
Press SELECT to perform the Paper Path Test.

If this does not duplex, or if you can't select duplexing, you've got a hardware problem.

Let us know what happens