U3521High Voltage System Interlock Circuit F
- Generic OBD-II code · Network
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- U3521 flags an unspecified fault in loop F, the sixth interlock loop. Extra loops often guard service openings rather than cables. FMVSS 305a defines the service disconnect as the device that switches the circuit off for checks and service and demands finger protection wherever it opens without tools; its interlock tells the controller when it is pulled. Covers can be watched without contacts too: GM patented an interlock switch built from a magnet and a reed relay that stays closed only while the magnet is near. No maker publishes which opening belongs to loop F. Check that the service plug is fully locked and that magnet covers sit correctly.
- What you may notice
- No ready mode after battery or service work
- High-voltage system message in the cluster
- Likely causes
- LikelyService disconnect not fully inserted or its lever not locked
- PossibleCover with interlock magnet missing, swapped or mounted out of position
- RareReed relay or switch contact of the cover interlock failed
- RareLoop F wiring damaged
- Repair
- Hard · Usually a job for a workshop.Typical cost: €50 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.