Shaun, before you did the Yosemite upgrade, did you make a full system backup (ala Carbon Cloner or SuperDuper)? If so, it might be worth restoring it or if you can get your hands on another Mac wo Yosemite and load the driver.
Seems waaaay too 'coincidental' for it to be the sensor.
You may also want to try reloading the driver from scratch (ie. remove the /Library/Printers/HP folder, reboot, reload)