Wrong forum, there are better forums for things like that, i dont know if it is allowed to link them here.

I did an immo of on a 156 0261204705 ages ago which should be the same hardware,
all i remember is that i based it off a immo off file for the 166 3.0 also with ME2.1.

There where two patterns which included a 0xDA byte which had to be replaced with 0x25.
Or was it 0x25 with 0xDA, i dont remember, but these are the "magic bytes" used on allmost all alfa gasoline ecus to signal IMMO allow/deny.

No, immo off works fine without touching the HC711E9 which emulates an eeprom iirc.

What you do is essentially patching the code in the flash from
if (wfsAllow == ALLOWED_0x25)
   allow_start_engine();

to

if (wfsAllow == NOTALLOWED_0xDA)
   allow_start_engine();

The side-effect is that it wont start anymore if the immo is completely ok ;-)

But you should fix up the checksum as it can come up later.
Flash checksum can be done delayed or every X afterruns (when ignition is off and ecu does a few seconds housekeeping)

0x487c from 0x25 to 0xDA should do the trick ;-)

Sorry, dont have any checksum tools for that old ecu anymore.
iirc i ran this for ages without doing the checksum.