Error E4 of HP Smart Tank 750 printer

[hakuuart]

Hello everyone, I need some help or advice regarding the HP Smart Tank 750 device. I have already checked for paper jams and anything stuck inside, as well as all the flex cables, but I still encounter an E4 error. We have taken it to customer service for inspection, but they don't know how to fix it. Is it possible that the issue lies with the main board or the sensor? I would really appreciate any guidance from those who have experienced this problem. Thank you!

[stj]

if E4 is the paper sensor then try soldering a wire to the connection on the mainboard so you can meter or scope it while it's running.
if it's optical then check the led is getting power and running

[hakuuart]

if E4 is the paper sensor then try soldering a wire to the connection on the mainboard so you can meter or scope it while it's running.
if it's optical then check the led is getting power and running

thank you, i will try this