U3512: High Voltage System Interlock Circuit B Performance

HVIL loop B is electrically OK but timing/pattern does not match expected behavior — possibly intermittent contact in a connector.

Symptoms

• Vehicle will not switch to ready mode or the HV system shuts down
• High-voltage system warning in the instrument cluster
• Charging may be blocked

Most likely causes

• Intermittent contact in HV connector
• HV connector, service disconnect or cover switch not fully seated
• Damaged interlock wiring or corroded terminal in the loop
• Fault in the module that drives and evaluates the interlock loop

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes with a hybrid-compatible scanner and write down the freeze frame data
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Make sure the battery cooling air intake is clean
4. Check the 12 V auxiliary battery, which can also trigger warnings
5. Leave any high-voltage measurements to a specialized repair shop

Good to know

• Repair difficulty: hard
• It can trigger limp mode.

What each character of U3512 means

• U: Communication network
• 3: Generic code: same on every make
• 5: Subsystem depends on the code range
• 12: Specific fault in that system

Frequently asked questions

Can I keep driving with code U3512?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code U3512?

The most common causes are: intermittent contact in HV connector; hV connector, service disconnect or cover switch not fully seated; damaged interlock wiring or corroded terminal in the loop.

Related codes

• U3516High Voltage System Interlock Circuit C Performance
• U351AHigh Voltage System Interlock Circuit D Performance
• U351EHigh Voltage System Interlock Circuit E Performance
• U3522High Voltage System Interlock Circuit F Performance
• U3526High Voltage System Interlock Circuit G Performance
• U352AHigh Voltage System Interlock Circuit H Performance
• U352EHigh Voltage System Interlock Circuit J Performance
• U3532High Voltage System Interlock Circuit K Performance

Sources

• github.com: dtc database
• en.wikipedia.org: Battery management system
• law.cornell.edu: 13 CCR 1968.2
• ti.com: tidud43a

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.