Hi,
I'd like to share some of my findings with regard to service mode operations on CLI 551/251 generation of printers.
I have tried to use service mode on 3 representants - MG5650, IP7250, MG7150. All 3 entered service mode after "standard" 5 click sequence - no problem with that.
1) My first try was on MG5650 with updated firmware using v3400 service tool -> printer locked, not possible to turn it off, I had to pull of of the socket. Service mode not available anymore.
2) Then I dowloaded v4720 service tool, which obviously has more functions and is more up to date (about info says "Canon XXX-2015") and tested on IP7250 with original firmware - THIS WORKS LIKE A CHARM !!! - several function were tested, including waste pad reset. So no special magic was needed, just newer Service tool and original firmware
3) I tested the same v4720 tool on MG7150 with updated firmware - Error 005 (this printer is not compatible with service tool) and again the only way to continue was to pull the printer off the wall socket. And again - no service mode anymore.
So it really confirms, that Canon prevents using older service tools by continuosly updating firmware in printers connected to internet. Of course you can refuse automatic udates in printer configuration, but in case you get an older printer of this generation from a "normal" user, you have the latest firmware inside with high probability - particularly in case of multifunction device with display, where Canon interactively offers new firmware to the user.
In case your printer does not want to enter service mode then it means somebody (sonner than you) already tried to play with this mode using an incorrect tool.
For the moment there is no information available how to re-enable access to service mode.
The only way around this is to dissasemble the printer, unsolder both EEPROM chips, read and store EEPROM data, solder them back. In future you can repeat the process and write previously stored data back to EEPROM once you get the message that waste pad is full. Of course this is not a solution for everyone, you have to have some knowledge, skills and tools to do it properly. And it will take some time.
As I don't like to give up easily I might go this way in future, I will post the results in case I really decide to do it.
Pavel