U3520High Voltage System Interlock Circuit E High
- Generic OBD-II code · Network
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- No ready mode, high-voltage warning shown
- Fault reappears directly after clearing
- Likely causes
- LikelyLoop E wire shorted to a 12 V supply
- PossibleWater or corrosion bridging a loop pin to a supply pin
- RareLoop source in the controller stuck on
- Repair
- Hard · Usually a job for a workshop.Typical cost: €80 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.