Network, generic (extended)
U3520 High Voltage System Interlock Circuit E High
Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible
U3520 means the fifth interlock loop sits above its permitted range, usually because a loop wire has picked up battery voltage or another low-voltage feed. Lear's patent for a pulsed interlock tester shows how to pin this down further: it measures current at generator and detector pins and, by comparing them, works out at which end the unwanted link to the 12 V side lies. Simpler controllers only see the raised level. No manufacturer has published U3520. In the workshop, test each loop E wire against 12 V with the plugs at both ends disconnected; wet or corroded connectors next to fused feeds are typical spots.
Likely causes
Loop E wire shorted to a 12 V supply
Water or corrosion bridging a loop pin to a supply pin
Loop source in the controller stuck on
Symptoms
- No ready mode, high-voltage warning shown
- Fault reappears directly after clearing
Repair estimate
DifficultyHardWorkshop job
Parts and labour€80–1,200
Time0.5–4 h
Typical range across vehicles; the real cost depends on what the testing finds.
Parts involved
hvil loop ehvil wiring
Related codes
References: patents.google.com, law.cornell.edu.