FortiGate 120G Dual PSU False Amber Alarm on Cold Boot
FortiGate 120G and 121G next-generation firewalls equipped with dual redundant AC power supplies report a false amber LED failure and high-priority SNMP alarms on PSU2 following a cold boot or power-cycle event due to I2C sensor initialization timeouts.
Technical Root Cause Analysis
The FortiGate 120G/121G hardware platform features dual hot-swappable AC power supply modules connected over a shared internal SMBus/I2C sensor bus. During system cold boot or sudden AC power restoration, the baseboard management micro-controller queries power supply telemetry registers (input voltage, temperature, fan RPM, output current).
In early FortiOS 7.4 releases supporting the 120G NPI release, the sensor polling thread initiates register interrogation 1200ms before PSU2 has completed its internal power rail stabilizing sequence. When the I2C read operation times out without an ACK response, the kernel latches PSU2 into a permanent "Fault / Input Lost" state. The chassis LED turns amber, SNMP trap fgTrapPowerSupplyFailure is fired, and syslog flags PSU2 as defective, despite the power supply delivering full, redundant 12V output.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.4 (120G NPI) | 7.4.2 NPI – 7.4.3 | 7.4.4 GA+ | Re-poll sensor via CLI or upgrade to 7.4.4 GA |
| FortiOS 7.2 | Not Applicable | Not Supported on 120G | 120G platform requires FortiOS 7.4 or later |
| FortiOS 7.0 | Not Applicable | Not Supported on 120G | Hardware generation not present on 7.0 |
Platform Impact: Exclusively affects FortiGate 120G and FortiGate 121G models with dual redundant AC power supply units installed. Single-PSU units and other G-series models (e.g., 90G, 200G) utilize distinct sensor bus layouts.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands to verify whether PSU2 is electrically operational and distinguish false I2C timeout alarms from genuine power supply hardware defects:
Diagnostic Commands
# 1. Query real-time chassis power supply sensor outputs
execute sensor list | grep -i PSU
# 2. Inspect low-level hardware device info for PSU 1 and PSU 2
diagnose hardware deviceinfo psu
# 3. Check PSU electrical parameters and rail status
diagnose hardware sysinfo psu
# 4. Review hardware sensor alarm history
diagnose hardware sensor statusRemediation & Workaround Steps (Teaser Preview)
Follow the complete step-by-step remediation guide in the interactive Ferrite platform:
Frequently Asked Questions
During cold system initialization, the Linux sensor daemon probes the internal I2C bus before the micro-controller on the secondary power supply (PSU2) has finished its internal power rail synchronization. When the I2C read times out, FortiOS flags PSU2 as missing or faulted, latching an amber front panel LED and sending false SNMP traps.
No. The power supply is functioning normally at the electrical and hardware layer, providing continuous 12V power and proper load balancing. The issue is strictly an I2C communication race condition during boot sequence initialization.
FortiOS 7.4.4 GA and higher introduce an asynchronous 3000ms debounce delay and multi-attempt retry loop on the I2C sensor bus during boot, preventing false alarms from latching before power rail stability.