BYD Seal Intermittent Failure to Enter OK (READY) State: Diagnosing an Intermittent Connection in the High-Voltage Interlock Loop Female Terminal

Fault Background

The Seal's "power-on" requires the vehicle's high-voltage system to complete self-checks first, among which the High-Voltage Interlock 1 (HVIL-1) loop is a safety ring series-connected across multiple high-voltage components—the service disconnect switch, the integrated smart rear drive assembly, and the floor wiring harness connectors are all in this loop. If any point in the loop has an intermittent connection, the OK (READY) state cannot be achieved, and the vehicle intermittently loses power. This article dissects a case of "intermittent prompt to check connector and no power output while driving"—the fault was ultimately traced to an oversized female socket on a terminal.

Fault Symptoms

Cause Analysis

1. Integrated smart rear drive assembly fault—abnormal interlock loop inside the rear drive assembly.
2. Powertrain domain controller fault—abnormal domain controller judgment.
3. Traction battery fault—abnormal interlock loop at the battery end.
4. Wiring harness fault—intermittent connection or open circuit in connectors along the interlock loop.

Diagnostic Procedure

1. Read and sort DTCs: After scanning with VDS2100, more than a dozen modules reported faults. After clearing DTCs, the remaining codes were U041681 (ESP received invalid data), C1B6602 (angle initialization error), C121100 (no calibration data, multi-function video controller), and P1A600 (High-Voltage Interlock 1, current fault), DMC input signal invalid (current fault).
2. Eliminate interference items: ESP and angle sensor functioned normally after calibration; the multi-function video controller calibration was unsuccessful, but it is unrelated to OK light illumination, so it was deferred—priority was given to High-Voltage Interlock 1.
3. Segment-by-segment loop measurement: Measure continuity along the High-Voltage Interlock 1 loop segment by segment. Pins K76B-9 and K76B-16 on the integrated smart rear drive assembly were not continuous; pins Bok48-1 and BoK48-4 on the service disconnect switch were not continuous; however, Bok48-1 to BJK03-24 was continuous, KJB03-24 to K76B-16 was continuous, and KJB03-25 to K76B-9 was continuous—the loop was mostly intact, pointing to the KJB03 connector area.
4. First disconnect/reconnect test: After disconnecting and reconnecting the KJB03 connector, the fault did not reproduce. Pins were inspected with no bending or backed-out terminals. Wiggling the connector and harness did not reproduce the fault, and the vehicle entered OK state normally.
5. Fault recurrence: After restoring the vehicle, the OK light failed to illuminate again. Wiggling the KJB03 connector and harness did not reproduce the fault, but disconnecting and reconnecting the KJB03 connector restored normal operation—a classic terminal contact issue.
6. Per-pin resistance test on female terminals: Re-inspected male and female pins visually; no backed-out terminals. Used a single male pin to insert into each female socket to test mating resistance. When testing pin BJK03-25, no resistance was felt—the female socket was oversized, causing an intermittent connection.
7. Repair verification: After replacing the floor wiring harness, the failure to illuminate the OK light was completely resolved.

Repair Plan

1. First read and clear DTCs, separating "current faults" from the dozen or more history/related faults, and lock onto High-Voltage Interlock 1.
2. Measure continuity along the interlock loop segment by segment, using "continuous/not continuous" to narrow the fault area to a single connector.
3. If visual inspection is inconclusive, use a single male pin to test the mating resistance of each female socket—only this method can detect an oversized socket causing an intermittent connection.
4. After confirming the oversized female socket, replace the floor wiring harness. After repair, wiggle the connector and harness to retest, and perform multiple power-on verifications.

Key Parameter Quick Reference

Safety Warnings

• The high-voltage interlock loop is series-connected across multiple high-voltage components. Before measurement, the vehicle must be powered down; do not disconnect or connect high-voltage connectors while energized.
• After removing the service disconnect switch (Bok48), store it properly to prevent accidental reinstallation leading to energized work.
• After repair, perform power-on self-check and road test to confirm normal interlock status and no "Check connector" prompt before returning the vehicle.

Repair Insights

The biggest trap in this case is that "eyes can lie": two visual inspections of the pins showed no abnormality, yet the fault persisted. Three lessons: First, the clear-and-separate method—among more than a dozen DTCs, only the "current fault" is worth pursuing first; Second, the segment-measurement method—the interlock loop is series-connected, and continuity measurements can compress the fault from "the entire loop" to "one connector"; Third, the resistance-test method—for faults like an oversized female socket causing intermittent connection, the resistance felt when inserting a single male pin is more honest than a microscope. For invisible faults, let tactile sensation bear witness.

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【This article is for reference only】