NVIDIA H100 SXM5 80GB Error Xid 79: GPU Fallen Off The Bus
NVRM Xid 79 indicates an unrecoverable PCIe communication loss between host/PCIe switch and the H100 SXM5 GPU module. Root-cause mechanisms include physical baseboard socket unseating, PCIe Advanced Error Reporting (AER) fatal receiver/replay timeouts, transient 12V/48V rail power sag, or thermal throttle lockout.
Hardware Diagnostic Matrix
Spec Ref #af6beca6Required Diagnostic Tools & Instrumentation
Multimeter Probing Targets & Circuit Tolerances
Bench Calibrated ±5%| Test Point / Circuit | Target Nominal Spec | Expected State | Fault Condition |
|---|---|---|---|
| Main Logic DC Bus | 3.3V / 5.0V DC (±0.1V) | Stable DC Potential | <2.9V (Brownout / Sag) |
| Thermistor / NTC Sensor | 10kΩ - 100kΩ @ 25°C | Smooth Thermal Curve | 0Ω (Short) or ∞ (Open) |
| Safety Ground / PE Continuity | < 0.5Ω Resistance | Solid Earth Bond | > 1.0Ω (Floating Chassis) |
Common Symptoms & Fault Manifestations
- Kernel dmesg logs 'NVRM: GPU has fallen off the bus' alongside PCIe AER Fatal/Uncorrected Error
- nvidia-smi hangs, outputs 'Unable to determine the device handle', or returns missing GPU index
- Active PyTorch/NCCL workload crashes with CUDA error: an illegal memory access was encountered or device unmapped
Root Causes & System Failure Modes
PCIe AER Fatal Error triggered by physical pin contact degradation or improper torque on the SXM5 socket interface
Under-voltage transient or power drop on the 48V-to-12V SXM baseboard buck converters or VRM stage under sudden di/dt load step
PCIe retimer/switch (Broadcom/Astera Labs) lockup or PCIe Gen5 signal degradation causing link width/speed down-negotiation failure
Cold plate thermal interface material (TIM) pump-out causing instant junction-temp runaway and thermal safeguard isolation
High-Voltage & Mechanical Safety Isolation
Prior to opening access panels, disconnecting terminals, or probing wiring, perform complete lock-out / tag-out (LOTO). Disconnect all AC mains, DC string isolators, and lithium battery banks. Allow internal smoothing capacitors at least 3 to 5 minutes to dissipate stored residual energy below safe threshold (<30VDC).
Step-by-Step Diagnostic & Resolution Procedure
AER Telemetry Extraction and Software Topology Reset
Capture diagnostic registers via terminal: run 'dmesg -T | grep -E "Xid|AER"' and 'lspci -vvv -d 10de: | grep -A 20 "DevSta\|AER"'. Attempt host-level secondary bus reset via sysfs: echo 1 > /sys/bus/pci/devices/<GPU_DBDF>/remove followed by PCIe bus rescan. If device fails to re-enumerate at Gen5 x16, proceed to physical chassis intervention.
SXM5 Baseboard Power and Retimer Cold Cycling
Initiate a complete AC cold boot: shut down the host OS gracefully ('shutdown -h now'), issue an auxiliary chassis power disconnect via BMC or physically cycle the AC PDU branch circuits, waiting 180 seconds to allow all SXM5 baseboard capacitors and PCIe retimers to discharge completely before re-applying power.
Physical Reseat & Torquing Sequence Verification
Isolate server on an ESD station. Remove the cold plate/heatsink and visually inspect the SXM5 surface-mount connector pins for micro-debris, flux residue, or bent contacts using 10x magnification. Re-seat the H100 module into the SXM5 baseboard socket using the strict cross-pattern torquing sequence specified by the OEM (typically 0.45-0.55 Nm on retention screws) to eliminate mechanical skew.
NVLink/PCIe Stress Diagnostics and Validation
Boot system and verify enumeration with 'nvidia-smi -q'. Run NVIDIA Validation Suite: 'nvvs -p test_pcie.py -d 3' and high-load stress testing with 'nvbandwidth' across host-to-device and device-to-host pathways to verify zero correctable/uncorrectable PCIe AER events during sustained 900W+ chassis power draw.
Hardware Reset & Fault Clear Sequence
Controller ProcedurePrevention & Routine Maintenance Guidelines
- Maintain strict server inlet temperatures between 18°C and 22°C to prevent excessive thermal expansion delta on SXM high-density pads
- Ensure host PCIe AER Error Recovery is enabled in UEFI/BIOS alongside 'Force Gen5' retimer equalization profiles provided by OEM firmware
Frequently Asked Diagnostic Questions
What does error code Xid 79 indicate on NVIDIA H100 SXM5 80GB?
NVRM Xid 79 indicates an unrecoverable PCIe communication loss between host/PCIe switch and the H100 SXM5 GPU module. Root-cause mechanisms include physical baseboard socket unseating, PCIe Advanced Error Reporting (AER) fatal receiver/replay timeouts, transient 12V/48V rail power sag, or thermal throttle lockout. In engineering telemetry, it indicates gpu fallen off the bus requiring circuit verification.
What are the primary triggers causing error Xid 79?
Diagnostic bench telemetry isolates the following common root causes: PCIe AER Fatal Error triggered by physical pin contact degradation or improper torque on the SXM5 socket interface; Under-voltage transient or power drop on the 48V-to-12V SXM baseboard buck converters or VRM stage under sudden di/dt load step; PCIe retimer/switch (Broadcom/Astera Labs) lockup or PCIe Gen5 signal degradation causing link width/speed down-negotiation failure; Cold plate thermal interface material (TIM) pump-out causing instant junction-temp runaway and thermal safeguard isolation.
How do I reset and clear NVIDIA error Xid 79?
Full AC power-cycle via Out-of-Band BMC: run 'ipmitool chassis power cycle' to completely drop standby rails, clear the PCIe bridge error registers, and force PCIe link training re-enumeration. If the error status repeats immediately after clearing, discontinue clearing and test circuit continuity.
Can I safely resolve error Xid 79 without specialized shop tooling?
This fault is graded as Advanced difficulty with Critical severity. Standard diagnostics require an insulated digital multimeter and appropriate hand tools. Always observe high-voltage isolation procedures.