After re-flashing old firmware and then back to the new 1.2.27 problem disappeared.
So it was a software-corruption thing again.
I'm not sure whether flash or eeprom area got corrupted (again) but it was not detected this time so it's dangerous.

I hope it is resolved now by sorting ignition cable misfires but I still suggest more checksum protection of all data areas.

Regards,
Gergely

Thanks Dave,

I did a test with a spare ECU using same 1.2.27 fw and config. It seems you are right, the replacement ECU is acting fine. Still it's very strange to me how such a software-related thing can go wrong while other functions look fine.
I'll consult with Marcell how to proceed with this ECU.

Thanks again!
Gergely

Hello Gergely,

I cannot reproduce this, i suspect there is some problem with your board (or atmega128) on this ecu. This can easily have been caused by the ignition noise / misfires to GND. The Flash corruption and magic "Safe Mode" engagement is also not normal behavior

I would advice to have the ecu checked before anything else.

Best regards, Dave