Network, generic (extended)

U3521 High Voltage System Interlock Circuit F

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Service disconnect not fully inserted or its lever not locked
Cover with interlock magnet missing, swapped or mounted out of position
Reed relay or switch contact of the cover interlock failed
Loop F wiring damaged

Symptoms

  • No ready mode after battery or service work
  • High-voltage system message in the cluster

Repair estimate

DifficultyHardWorkshop job
Parts and labour€50–1,200
Time0.3–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

hvil loop fhv connectors

Related codes

U3522U3523U3524U351DP0A0A

References: law.cornell.edu, patents.google.com.