Canon s400 doesnt print only blinks eight times

The tank is situated on the RHS.
The Print Head rests there when not in use.
Open the flap to expose the Print Head,slide the Print head across to the left.
On the area where the Print head rests, there is a black plastic square of plastic which contains 2 narrow vertically positioned ink pads.This is also spring loaded.from underneath.
The pad on the left is for BLACK the other Color.
These become saturated with ink during the cleaning process.
I suspect the Print Head has a built in sensor to detect overloaded waste tank.
The pad can be removed this by hooking it out, sideways.
Unless the pads are leaking ink everywhere I wouldn't remove it.
Use Cotton Buds to soak up the excess ink. I used about 15.

Then.....
Turn off printer
Hold down RESUME button and press POWER button.
Keep holding down POWER button and release RESUME button.
Press RESUME button 2 times, then release BOTH buttons.
The GREEN light will flash and stop flashing.
When the GREEN light goes out, press the RESUME button 4 times.
Press the POWER button and the printer should turn OFF
If not press the POWER button once more.

My experience was when turning the printer on at a later date the POWER and RESUME lights still flashed.
All I did was press the RESUME button and the printer operated as normal.

Refer to the manual or online to find out how to properly execute the waste tank procedure. Also the manual should tell you how to reset the code
 after you clean the tank. The tank is located on the bottom, it is a
 felt pad that you have to rinse out. There is lots of ink involved so
 it can get messy.