undef_instr — Software
The kernel hit an undefined instruction. iFixit and Elektroda mainly see a software problem (system or incompatible app); if the fault persists after a restore, suspect the RAM or NAND and their circuits.
iPhone
Software (iOS / incompatible app); otherwise RAM or NAND
Medium confidenceVerified sourceSoftware
First check
Update iOS and apps, then restore; if it persists, RAM/NAND diagnosis.