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U3544 — High Voltage System Interlock Circuit O High

Manufacturer-specificNetwork / communication

U3544 indicates that the vehicle's high‑voltage interlock (HVIL) circuit is reading an abnormal high voltage, typically because the circuit is open or shorted to power. The HVIL is a safety loop that the powertrain control module (PCM) monitors to ensure the high‑voltage system is properly connected and safe. When the module detects the line is high instead of the expected low (ground) level, it sets this fault code.

Symptoms

What causes it — most likely first

1. Damaged or corroded HVIL wiring/connectors

Chafed, pinched, or corroded wires or loose connectors can create an open circuit, causing the line to read high when the PCM expects a low reference.

2. Failed HVIL relay or contactor

If the interlock relay contacts are stuck open or have high resistance, the circuit cannot be pulled low, resulting in a high reading at the PCM.

3. Faulty high‑voltage interlock switch or sensor

Switches that are supposed to close the HVIL loop (e.g., hood latch, service disconnect) may be damaged or mis‑adjusted, leaving the circuit open.

4. High‑voltage battery internal fault

Internal short or open within the battery pack can prevent the HVIL from being grounded, leading to an abnormal high voltage on the line.

5. Control module (PCM/HV controller) internal failure

In rare cases the module itself may incorrectly read the HVIL voltage, but this is usually a last‑resort diagnosis after all other circuit components have been verified.

How it's diagnosed

  1. Connect an OBD‑II scan tool, record the fault code and any related freeze‑frame data.
  2. Visually inspect the HVIL wiring harness and connectors for damage, corrosion, or looseness.
  3. Measure the voltage on the HVIL circuit with a digital multimeter; the line should be low (near ground) when the interlock is closed. A persistent high voltage indicates an open or shorted circuit.
  4. Test the HVIL relay/contactor for proper operation (actuation and contact resistance) using the manufacturer’s procedure.
  5. Check the individual interlock switches (e.g., service disconnect, hood latch) to ensure they close the loop correctly.
  6. If the circuit tests OK but the fault persists, verify the PCM/HV controller’s reference voltage and consider module replacement as a final step.

Code facts & related codes

SystemNetwork / communication
StandardizationManufacturer-specific (P1/B1/C1/U1)
Related codesU3540 — High Voltage System Interlock Circuit U3541 — High Voltage System Interlock Circuit U3542 — High Voltage System Interlock Circuit U3543 — High Voltage System Interlock Circuit U3545 — High Voltage System Interlock Circuit U3546 — High Voltage System Interlock Circuit

Frequently asked

Is it common to replace the powertrain control module (PCM) for a U3544 fault?

No. The PCM rarely fails on its own for this code. In most cases the fault is in the wiring, connectors, relay, or switches of the high‑voltage interlock circuit. Replacing the PCM should be considered only after the external circuit has been thoroughly tested and cleared.

Can I drive the vehicle with U3544 set?

Driving is not recommended because the high‑voltage system may be disabled or may not engage properly, leading to reduced power or a stall. The vehicle will often enter a limp‑home mode and may not start.

What does “O High” mean in the code description?

The “O” typically stands for “Open” (or open‑circuit). The code indicates that the high‑voltage interlock circuit is reading a high voltage when it should be low, suggesting an open circuit or a short to power somewhere in the loop.

Can a low‑voltage battery issue cause U3544?

The code pertains specifically to the high‑voltage interlock circuit, not the 12 V battery. However, if the low‑voltage system powers the HVIL relay coil, a weak 12 V supply could cause the relay to stay open, producing the same symptom.