U3548: High Voltage System Interlock Circuit P High

What it means

HVIL loop P voltage above expected range. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

The high-voltage interlock loop (HVIL) is a safety circuit that lets the vehicle monitor whether designated high-voltage connectors and service access points are properly closed. This code indicates that the monitored P interlock circuit voltage is higher than the control module expects; an open loop or a fault in the circuit or its reported data may interrupt high-voltage operation.

Common causes

Most likely first.

1. Interlock loop open
2. Gateway module routing CAN messages incorrectly
3. CAN terminating resistor (120 Ω) drifted out of spec

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Read all modules' codes and freeze-frame data first, noting whether U3548 appears with other communication or high-voltage interlock faults.
2. Check whether the fault is current and whether warnings or reduced-power behavior began at the same time; this helps distinguish an active interlock problem from a stored event.
3. With the vehicle powered down according to its safety instructions, visually inspect accessible P-loop connectors and harness sections for looseness, damage, moisture, or corrosion without disconnecting high-voltage components.
4. Have a qualified technician check the P-loop signal and wiring continuity against the vehicle manufacturer's procedure, since high-voltage interlock circuits can be hazardous.
5. If the loop checks out, investigate gateway communication and the relevant CAN network, including termination only where the manufacturer's diagnostic procedure calls for it.

Common fixes

• Secure or repair an accessible low-voltage interlock connector or damaged harness section identified during inspection.
• Repair an open or high-resistance fault in the P interlock loop using the manufacturer's repair procedure.
• Correct a confirmed gateway configuration or CAN routing fault.
• Replace a failed gateway or other confirmed module only after its power, grounds, wiring, and network have been checked.

Can I keep driving?

Avoid driving if the vehicle reports a high-voltage system warning, reduced power, or shutdown risk; the interlock fault can disable propulsion or other high-voltage functions. Do not handle or bypass high-voltage interlock components yourself.