NVIDIA RTX 4090 GPU fault (Xid 79 - GPU has fallen off the bus) caused by a defective PCIe riser cable on a mining-style GPU mount, resulting in intermittent PCIe link degradation.
The GPU lost contact with the PCIe bus - a hardware connectivity fault. Xid 79 is the NVIDIA error code for "GPU has fallen off the bus", almost always indicating a physical connection issue rather than a software problem.
High confidence (87%). Xid 79 with progressive PCIe link degradation is a near-conclusive hardware signature. 13% uncertainty because riser cable vs slot vs GPU card itself requires physical inspection to confirm.
PCIe link downgraded: Gen4 x16 → Gen3 x8
First sign of PCIe instability - link negotiating lower speed/width
NVRM: Xid 13 - Graphics Exception: ILLEGAL_INSTR_ENCODING
GPU generating faults from unstable PCIe reads
PCIe AER: Correctable error: Bad TLP - count: 847
PCIe packet corruption accelerating rapidly
NVRM: Xid 79 - GPU has fallen off the bus
Fatal GPU disconnect - driver can no longer communicate with hardware
Kernel panic - not syncing: GPU fault recovery failed
Kernel unable to recover from GPU loss, forced reboot
gpu_nvidiaweight96%
NVRM: Xid (PCI:0000:01:00): 79, pid=0, GPU-00000000, Channel 00000000Xid 79 = GPU has fallen off the bus. pid=0 confirms hardware fault, not application crash.
dmesgweight91%
pci 0000:01:00.0: PCIe link downgraded from Gen4 x16 to Gen3 x8 to Gen2 x4Progressive link speed degradation over 2 hours before crash - classic riser cable failure signature
dmesgweight89%
pcieport 0000:00:01.0: AER: Correctable error count: 847 in last 180s847 PCIe packet errors in 3 minutes - riser cable or slot failing
gpu_nvidiaweight82%
NVRM: Xid 13 count=23 prior to Xid 79Progressive GPU fault escalation - 23 illegal instruction errors preceded the bus disconnect