HP P2015dn jam light comes on only when using the back exit.

[AnyNiceName]

It began when some paper bits prevented tray 1 from drawing in an envelope. If I pressed the envelope in a bit, then it would go through but the paper jam light comes on after. Did not matter as I would just turn on to do an envelope then off after, so on rebooting the error is removed, HP programming I guess. However the other day doing a clean up I noticed a wee bit of paper blow out from the toner bay, low and behold the stoppage in tray 1 is gone! However the jam light is till coming on.  Hmm on investigating when I sent a test page from the printer buttons to the back the light did not come on, so all the suggestions telling me the sensor is blocked, more paper bits?,  are mistaken. It took some effort to eliminate the sensor, including taking the printer outside to the workbench and giving it a blow down with my workshop pressure air hose. Still lighting up! If not the machine it HAS to be the PC print commands, in my case all PCL. When investigating that I notice several settings on HP Printer popups do NOT apply to me. So it HAS to be the driver software, incorrectly installed etc by Microsoft when I upgraded from Windows 10 64 Bit. Sure enough after completely removing the MS installed driver - has to be done in Start  Control Panel ...Printer Management applet, I started from fresh, downloaded the latest PCL driver from HP and installed that. Hey presto the error is gone! Thanks HP for keeping my legendary HP P2015dn working! Just under 6000 pages since new, bought in the early 2000s.  

[Repairatrooper]

If the light comes on only when printing from a program that would suggest that the paper size settings of the job you are sending do not match the paper size settings on the printer itself.  For example, if you have letter sized paper loaded and you send a job that is not 8.8 x 11, the printer will assume a jam because sensor triggering will happen at the wrong time.