Network, generic (extended)

U3537 High Voltage System Interlock Circuit L Low

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U3537 is logged for an insufficient reading on loop L. The cause may lie outside the loop itself. In a Volvo patent with paired loops for trucks with separate high-voltage buses, neither loop is closed before the current sources start; one loop only closes once the source of the other energises the coil of an isolated relay. If that partner source stays off, or the relay coil or its contacts fail, the dependent loop reads as open though its own plugs are fine. With such a design, a low result on loop L calls for checking the codes and source of the coupled loop and the relay before any wiring is opened. No maker documents U3537; the contactors behind loop L stay open.

Likely causes

Plug within loop L disconnected or wire shorted to ground
Source of a coupled loop not running, so the coupling relay stays open
Coupling relay between loops with failed coil or contact

Symptoms

  • Vehicle or one high-voltage bus does not become ready
  • Interlock codes for more than one loop stored at once

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–1,500
Time0.5–4 h

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

Parts involved

hvil loop lhvil wiring

Related codes

U3535U3536U3538U3533P0A0C

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