FortiGate 100F Power Supply Status False Alert after Firmware Upgrade to 7.4.1
Upgrading FortiGate 100F and 101F firewalls to FortiOS 7.4.1 triggers spurious "PSU2 Failed or Unplugged" alarms and SNMP traps due to an uncalibrated I2C register offset in the hardware sensor monitoring daemon.
Technical Root Cause Analysis
The FortiGate 100F and 101F hardware appliances feature dual hot-swappable AC power supply units monitored by the kernel environment sensor subsystem (psu_mon) via an internal I2C bus (/dev/i2c-1 at address 0x58). The daemon polls telemetry registers every 10 seconds to record voltage, amperage, fan tachometer readings, and the active power good (PWR_OK) signal.
In FortiOS 7.4.1 Build 2463, an updated sensor table intended for higher-end appliances was inadvertently applied to the 100F platform. This introduced an 8-bit register address shift when querying the auxiliary standby voltage rail (3.3V VSB) of PSU2. When the raw sensor reading experiences routine nominal AC line variance (±0.05V), the driver misinterprets the shifted byte as an active zero-voltage fault. FortiOS immediately logs logid="0100022131" msg="Power supply 2 is unplugged or failed", lights the red chassis warning indicator, and transmits spurious SNMP failure traps, despite the power supply operating at 100% capacity.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.4 | 7.4.1 Build 2463 | 7.4.2+ | Filter SNMP trap or upgrade to 7.4.2 |
| FortiOS 7.2 | Not Affected | 7.2.x Branch Clean | Correct I2C sensor map verified |
| FortiOS 7.0 | Not Affected | 7.0.x Branch Clean | Legacy sensor driver unaffected |
Platform Impact: Exclusively affects FortiGate 100F and FortiGate 101F physical appliances running FortiOS 7.4.1 Build 2463 with redundant dual power supplies connected.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these commands to verify the physical status of the power supplies and inspect current sensor telemetry:
Diagnostic Commands
# 1. Query environment sensor readings for power supply entries
execute sensor list | grep -i psu
# 2. Inspect raw low-level hardware power supply telemetry
diagnose hardware deviceinfo psu
# 3. View system power state and dual-PSU input voltages
diagnose hardware sysinfo power
# 4. Check low-level Linux kernel power supply procfs registers
fnsysctl cat /proc/power_statusRemediation & Workaround Steps (Teaser Preview)
Follow the complete step-by-step remediation guide in the Ferrite interactive platform:
Frequently Asked Questions
No. If the physical LED on the back of PSU2 is solid green and the issue began immediately following the FortiOS 7.4.1 upgrade, the hardware is fully operational and an RMA will not resolve the issue.
No. The bug is tied to the register address offset queried for slot 2 in the FG-100F chassis driver, so slot 2 will continue reporting failed regardless of which physical PSU module is installed.
FortiOS 7.4.2 GA permanently restores correct I2C register addressing for FG-100F/101F chassis sensors.