Err code on kenwood kdc355u unit have power but no sound, help please

check the wire harness connections, sounds like you have KOER power hooked to KOEO. Or in other words, you have power to the radio with the key off that should connected to key on power

You can enter a 4-digit security code as an option after purchase of the unit. If you do so, make sure it is noted down somewhere. If you did select a code (or the previous owner has done so) this must be used when the unit is removed from the vehicle, or the constant power source is disconnected from the unit. When constant power is restored to the unit, the security code must be entered before the unit becomes operational. If the code is lost or forgotten I am afraid the unit has to be re-coded by a Kenwood dealer.
Hope this helps, Good luck with it.
However here is a link for a tip as posted by robotek, therefore all credit should go to him, Regards.
http://www.fixya.com/support/r538828-security_unlock_code_kenwood_most_models

You can enter a 4-digit security code as an option after purchase of the unit. If you do so, make sure it is noted down somewhere. If you did select a code (or the previous owner has done so) this must be used when the unit is removed from the vehicle, or the constant power source is disconnected from the unit. When constant power is restored to the unit, the security code must be entered before the unit becomes operational. If the code is lost or forgotten I am afraid the unit has to be re-coded by a Kenwood dealer.
Hope this helps, Good luck with it.

You can enter a 4-digit security code as an option after purchase of the unit. If you do so, make sure it is noted down somewhere. If you did select a code (or the previous owner has done so) this must be used when the unit is removed from the vehicle, or the constant power source is disconnected from the unit. When constant power is restored to the unit, the security code must be entered before the unit becomes operational. If the code is lost or forgotten I am afraid the unit has to be re-coded by a Kenwood dealer.
Hope this helps, Good luck with it.

Readings You May See When Unit is AC Powered
Display Shows Alarm Sound Unit Status Recommended Action
Steady "Err" "Chirp" every Unit malfunctioning when Call KIDDE customer
and flashing dot 30 seconds AC powered service at 1-800-880-6788
Readings You May See When Unit is on Temporary Battery Backup
Display Shows Alarm Sound Unit Status Recommended Action
Flashing dot None Normal battery-only operation - Plug in to AC as soon as
unit will show reading only if it possible to conserve battery
senses CO
Flashing dot and "Chirp" every Unit malfunctioning when Call KIDDE customer
"Err" 30 seconds battery powered service at 1-800-880-6788

Hope this helps. Manual is here:
http://www.detectorsandalarms.com/images/pdf/Manual_KN-COPP-3_900-0100en.pdf

If this helps you resolve your problem, please consider giving a good rating and a quick thanks or good job.It will help me as well Thank you.

Get in the service mode *28* and there under function, clear, err.

If not replace the fixing film unit

try the sequence within that number starting with
0000 and so on

this is caused from the pickup needing to be cleaned(lazer) remove deck remove small phillips screw on top plate use a damp q-tip (alcohal) gently rub the lazer top with the q-tip allow to dry aprox 10min reasemble and enjoy. Also make sure u don't need a security code for your radio before u unhook the power plug!!!!

Bu the sounds of it, you will only need to enter the mask code if the memory wire becomes disconnected, so I would assume that you have your power(red) and memory/constant 12v backword, or both hooked up to the power wire

If you take it out and install it again it will remain locked. Can you contact supplier and find out the code? If not take the serial number off the unit and call Kenwood, They can help you out.