HP Deskjet 6127 Color Inkjet Printer - Manual (Page 111)

paper jam occurs inside the duplexer
Follow the instructions for clearing a paper jam in the printer is not printing section.
automatic option for two-sided printing cannot be selected in the Features tab
The printer driver may not be configured to use the duplexer .
Follow these steps to configure the printer driver to use the duplexer:
1. Open the Configuration dialog box in Printer Assistant, then click the configure
button .
2. Click the Automatic Two-Sided Printing Accessory check box.
3. Click the Apply button.
4. Close the Configuration dialog box.
5. Open the Printer Properties dialog box, then select the Features tab.
The Automatic option now appears beneath the Two-Sided Printing option.