Network, generic (extended)

U353C High Voltage System Interlock Circuit M High

Check engine lightOn

EmissionsNot relevant

Drive cycleNot required

Limp modePossible

U353C indicates that loop M exceeds its upper threshold. A loop built from known resistances can be checked point by point. In Tesla's Model S tech note the battery management sends a constant 20 mA through four resistance stages of 60 ohms, 240 ohms in total, so about 4.8 V appears at the start and the value falls to 3.6, 2.4 and 1.2 V at the following connectors. Back-probing each point and comparing it with these steps shows in which section extra resistance or foreign voltage sits; resistance to ground with the loop off runs in matching 60-ohm steps. A wiring diagram with such reference values makes loop M quick to trace. No maker documents U353C itself.

Likely causes

Corroded or loose contact adding resistance in one section of loop M

Loop M wire in contact with a 12 V supply

High-voltage component with internal loop resistance out of specification

Symptoms

Vehicle does not become ready
High-voltage warning returns after clearing

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 mhvil wiring

Related codes

U3539 U353A U353B U3538 P0A0D

References: static.nhtsa.gov, law.cornell.edu.