This is for a 1997 Honda Accord, but I'd guess that it holds true for a wide variety of Honda's of comparable vintages. It involves the stock combo radio and CD player that came with the car.
This information is out there already, but I had to search several blogs and the information was not clear, and the posters were "like you oughta try this dude, might work". Here is what actually happened and the simple fix.
My son left his lights on and killed his battery. I charged the battery but neglected to check the radio and it had gone into theft protection mode and was displaying "Code".
When it displays "Code" you are supposed to pull out the paperwork you got when you bought the car, and there is a white card like a credit card with a 5 digit number (each digit below 3) that you type into the preset buttons to unlock it. This is to discourage theft of radios. (Must have worked because nobody does this lame crime anymore). Many owners write this number on the user manual, which I had done.
However, I wasn't around and he and his teenage friend tried entering random numbers, not knowing what was going on. The radio went into lockdown mode after several repeated failed tries. Now the radio displays "Err 3" and wont accept a code. It is a doorstop.
What didn't work
Several posts suggested going to the dealer, or pulling the radio and getting the serial number off the back and calling Honda, and all kinds of other garbage that involved hours of work and dealing with customer service people. All for a teenagers 15 year old car. Ugh.
One post suggested turning off the car, pressing 1 and 6 simultaneously and holding, and turning the car back on. This sounded like the sort of automobile trick that would work, but it did not. You could try it but it didn't work for me.
The easy fix
Fix was easier than I imagined.
Simply open the hood, with the car off, and disconnect the negative (black) terminal of the battery. Takes a wrench and the nut was pretty rusted on this old car. Leave it off for a few minutes. I got in the car and turned the ignition on and back off as an extra step to be sure I drained any residual charge that might be keeping the radio alive.
Reconnect the battery, tighten the nut and turn the car on. The radio is back to saying "Code". Hooray! Err 3 message is gone and it accepts input. Entered the code from the radio card and all is good now. Teenagers can listen to the radio again instead of trying to play music from their phones while driving.
Now, later I thought, I could probably have just pulled the fuse for the radio from the fuse panel under the dash instead of disconnecting the battery. I did not try that. Sometimes it is hard to know which fuse to pull and if that fuse will remove the standby power as well. But I recommend trying this if you don't have tools or don't want to mess with the battery. It will probably work if you pull the right fuse.