A short to voltage on the input power B circuit wiring; A faulty control module that is internally shorted and feeding voltage back onto the circuit; A defective voltage regulator or alternator overcharging the system; Corroded or damaged connectors allowing voltage to bleed onto the circuit; A failed power supply relay or fuse box component supplying excessive voltage; Water intrusion into a module or connector causing unintended voltage paths; Aftermarket accessories improperly wired into the power circuit

DTC codes / U35xx · Network communication / U3506

U3506Control Module Input Power B Circuit High

Q: What does code U3506 mean?

Control Module Input Power B Circuit High. U3506 is a generic network communication code indicating that a control module has detected the input power circuit for a particular module or subsystem is higher than the expected voltage range. The computer is seeing voltage on the B circuit that exceeds the normal operating threshold, which can be caused by a short to voltage, a faulty power supply, or a module internal fault.

U3506 is a generic network communication code indicating that a control module has detected the input power circuit for a particular module or subsystem is higher than the expected voltage range. The computer is seeing voltage on the B circuit that exceeds the normal operating threshold, which can be caused by a short to voltage, a faulty power supply, or a module internal fault.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaNetwork communication

Symptoms you may notice

Illumination of the check engine light or service vehicle soon message
Often no symptoms besides the warning light
Intermittent electrical glitches such as flickering lights or erratic gauge behavior
One or more electronic features may stop working or behave unpredictably
The vehicle may enter a reduced power or limp mode in some cases
Battery drain or repeated blown fuses if the high voltage condition persists

Common causes, most likely first

A short to voltage on the input power B circuit wiring
A faulty control module that is internally shorted and feeding voltage back onto the circuit
A defective voltage regulator or alternator overcharging the system
Corroded or damaged connectors allowing voltage to bleed onto the circuit
A failed power supply relay or fuse box component supplying excessive voltage
Water intrusion into a module or connector causing unintended voltage paths
Aftermarket accessories improperly wired into the power circuit

How to diagnose and fix it

Use a scan tool to read all stored codes and freeze-frame data to confirm U3506 and identify the affected module
Check the battery and charging system voltage with a multimeter to rule out overcharging
Inspect the wiring harness and connectors for the input power B circuit for signs of shorting, corrosion, or damage
Disconnect the affected module and measure voltage on the B circuit to see if the high voltage is present without the module
Test the power supply relay and fuses for proper operation and replace any faulty components
If voltage is normal at the connector but the code persists, suspect an internal fault in the control module
Clear codes and perform a road test to verify the repair and ensure the code does not return

What a high voltage code really means

A high voltage code on an input power circuit often points to a charging system fault rather than the module itself, so always check alternator output first.
Intermittent shorts to voltage can be caused by wire chafing against a metal bracket, which may only show up when the vehicle is moving or vibrating.
Some modules will set this code if the power circuit sees voltage from a backfeed through a faulty ground, so ground integrity is just as important as the power side.
Aftermarket alarms or remote starters tapped into the power circuit are a common culprit that technicians overlook when diagnosing U3506.
Clearing the code without fixing the underlying high voltage condition can lead to repeated module failures or damage to sensitive electronics.

Can I keep driving with U3506?

If the only symptom is a warning light and the vehicle drives normally, you can drive cautiously to a repair shop, but avoid long trips and monitor for electrical glitches. If the vehicle enters limp mode or multiple systems fail, stop driving and have it towed to prevent further damage.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.