BYD Qin PLUS DM-i Intermittent Complete Power Loss: Troubleshooting a Poor Contact in the Low-Voltage Main Circuit

Fault Background

The Qin PLUS DM-i's low-voltage power supply is jointly provided by the 12V lithium iron phosphate battery, positive terminal fuse box, front compartment power distribution box, and the left/right domain controllers. Any "invisible" poor contact point can suddenly cause a complete vehicle power loss after normal driving—and the power loss is often intermittent, self-recovering, and without DTCs, making it one of the most challenging repair cases. This article breaks down the complete diagnostic approach for a real case.

Fault Symptoms

Cause Analysis

1. Low-voltage lithium iron phosphate battery fault—increased internal resistance or capacity degradation, voltage drop under high current demand.
2. Poor contact in positive terminal fuse box / front compartment fuse box—loose fuse blades or oxidized terminals.
3. Main power fuse and circuit fault—any open point in the main power feed lines to each module.
4. On-board power supply DC not generating—DC-DC output interrupted, 12V battery only discharging, not charging.
5. Ground point fault—poor contact in the negative return circuit, equivalent to an "open circuit in the entire loop."

Stored/History DTC Analysis

After the vehicle entered the shop, multiple modules had stored/history DTCs. Although the current state is normal, they are important "crime scene evidence":

Interpretation: Multiple unrelated modules "blinked" simultaneously, and "12V power supply output too low" appeared—this typically points to a transient open circuit in the low-voltage main power supply circuit, not a single module failure.

Diagnostic Procedure

1. All routine checks normal: In OK (READY) state, measured 12V battery at 13.9V, OFF state 13.2V, DC generation normal; positive terminal fuse box and front compartment fuse box terminals not loose; domain controller positive busbar bolts and all ground points normal.
2. Road test could not reproduce: Bumpy roads, hard acceleration/deceleration, simulating customer's stop-and-P scenario—all failed to reproduce—"currently normal" is the biggest enemy of such faults.
3. Change approach, actively induce the fault: In OK (READY) state, perform wiggle/pull tests (vibration/wiggle test) on the distribution wiring, fuses, and ground points one by one while observing the vehicle state.
4. Narrow down the suspect: Pulling the left domain controller wiring harness reproduced the complete power loss; after repeated verification, the suspect was the "left domain controller 200A fuse small battery terminal" wiring.
5. Disassembly confirmed root cause: Although the terminal nut was tight, foreign material was trapped behind the terminal—after removal, it was found that paper tape was sandwiched between the terminal and the busbar, causing poor contact. Manually disconnecting this terminal reproduced the power loss 100%, confirming the root cause.
6. Repair and verify: Cleaned the foreign material, reconditioned the terminal, and reinstalled it properly; wiggling the harness no longer caused power loss, and the fault was resolved.

Repair Plan

1. Remove the left domain controller-related wiring harness and extract the 200A power terminal contaminated by paper tape.
2. Clean foreign material from the terminal and busbar contact surfaces, check the terminal spring clip pressure, and replace the terminal if necessary.
3. Retighten the nut to the specified torque, and perform a terminal pull-back test after installation.
4. Use the vibration/wiggle test (vertical force + lateral force on the harness) to re-verify that the power loss no longer occurs.

Key Parameter Quick Reference

Safety Warnings

• The domain controller main fuse circuit can carry up to 200A; before disassembly, the vehicle must be powered down and the 12V battery negative terminal disconnected.
• Before restoring power, ensure all high-voltage interlocks and low-voltage connectors are fully seated to prevent secondary intermittent connections.
• When wiggling harnesses, avoid rotating parts and high-temperature components to prevent personal injury.

Repair Insights

Intermittent power loss faults are most daunting when "the car is fine." The lessons from this case are threefold: First, read stored/history DTCs—multiple modules "blinking" simultaneously is itself a clue; Second, use the vibration/wiggle test to actively reproduce—simulate bumpy conditions by applying vertical and lateral forces to the harness, letting the transient open circuit reveal itself; Third, always inspect terminals during disassembly—a tight nut does not equal good contact; foreign material behind the terminal, oxidation, and backed-out terminal pins are the real culprits. Remember: When you can't measure it, "manufacture" the fault to make it appear.

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【This article is for reference only】