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U3533 — High Voltage System Interlock Circuit K Low

Manufacturer-specificNetwork / communication

U3533 indicates that the high‑voltage interlock circuit K is reading a voltage lower than the expected value. The interlock is a safety loop that confirms high‑voltage components are properly connected; when the circuit is low the control module disables the HV system to prevent a shock hazard. The code is typically set when the circuit is shorted to ground, open, or when the interlock switch/relay fails.

Symptoms

What causes it — most likely first

1. Damaged wiring or connectors in circuit K

Chafing, pinched harness, corroded pins, or poor terminal contact can create a short to ground or an open, pulling the circuit low.

2. Failed HV interlock switch or relay

A faulty interlock switch may not toggle correctly, causing the circuit to remain low even when the connectors are seated.

3. Poor ground or reference‑voltage issue

If the ground path for the interlock circuit is compromised, the module sees a low voltage condition.

4. High‑voltage connector or terminal problem

Bent, burned, or oxidized contacts in the HV connector can interrupt the interlock loop.

5. Battery‑management (BMS) or HV control‑module internal fault

Rarely, the control module itself may fail to read the interlock correctly, but this is a last resort diagnosis after wiring is verified.

6. Intermittent harness movement or vibration damage

Repeated flexing of the HV harness can eventually break conductors or insulation, especially near moving components.

How it's diagnosed

  1. Retrieve all stored DTCs; note any additional U‑codes that point to other HV interlock circuits or network issues.
  2. Locate the high‑voltage interlock circuit K on the vehicle wiring diagram and identify the associated connectors, switches, and the BMS/HV control module.
  3. Perform a visual inspection of the harness and connectors for damage, corrosion, or loose terminals.
  4. Measure the resistance of the interlock loop with a multimeter (with HV system de‑energized) and compare to the manufacturer’s specification.
  5. Check for shorts to ground and verify proper reference voltage at the control module’s input pin.
  6. If the wiring and interlock switch test OK, substitute a known‑good BMS/HV control module or use a factory scan tool to monitor the interlock status before condemning the module.

Code facts & related codes

SystemNetwork / communication
StandardizationManufacturer-specific (P1/B1/C1/U1)
Related codesU3530 — High Voltage System Interlock Circuit U3531 — High Voltage System Interlock Circuit U3532 — High Voltage System Interlock Circuit U3534 — High Voltage System Interlock Circuit U3535 — High Voltage System Interlock Circuit U3536 — High Voltage System Interlock Circuit

Frequently asked

Is it safe to drive the vehicle with a U3533 code?

Usually not. The high‑voltage system is disabled when the interlock circuit is low, so the vehicle may run in a limited‑power or conventional‑engine mode. It is advisable to have the fault diagnosed promptly to avoid being stranded or damaging other HV components.

Can a bad PCM/ECM cause this code?

In most cases the fault lies in the wiring or the interlock switch itself. The PCM/ECM is rarely at fault; replacing it as a first step is usually unnecessary and expensive. Verify all external circuitry before considering a control‑module replacement.

Will the code clear on its own after a reset?

If the underlying problem is intermittent, the light may go out temporarily, but the code will return as soon as the condition recurs. Proper repair of the wiring or interlock component is required for a permanent fix.

What are typical repair costs for a U3533?

Costs vary by vehicle and labor rates, but the most common repairs involve harness inspection and repair or replacement of the interlock switch/relay, which are generally less expensive than replacing the high‑voltage control module.