Network, generic (extended)
U3524 High Voltage System Interlock Circuit F High
Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible
U3524 indicates a level on loop F above its upper threshold. Pulsed test schemes recognise this easily: if the loop is driven with a fixed square wave, as in Corun's 100 Hz battery design, a wire stuck at 12 V returns a flat level without any edges, which no intact connection could produce. Texas Instruments rates its interlock reference design to survive a direct connection to the 12 V battery, and it offers a third, high-impedance output state for diagnosis. No manufacturer publishes U3524. Look for chafing against fused 12 V wires and for low-voltage plugs in which a loop pin sits right beside a supply pin.
Likely causes
Loop F wire chafed against a fused 12 V wire
Loop pin bridged to a neighbouring supply pin by corrosion or a bent terminal
Source driver for loop F no longer switches off
Symptoms
- High-voltage system remains off, vehicle not ready
- Code returns at once 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 fhvil wiring
Related codes
References: patents.google.com, ti.com.