Geely Emgrand EV450 AC Charging Failure: On-Board Charger Thermistor Failure Repair

Fault Background

The AC charging function of the Emgrand EV450 relies on the on-board charger (OBC) — the AC power from the grid must first be rectified into DC by the OBC before it can charge the traction battery. The rectification process inevitably generates heat, so the OBC has a dedicated temperature detection circuit inside. When the thermistor (NTC) responsible for temperature measurement fails, the OBC becomes "blind" to its own temperature and, as an overheat protection measure, directly disables the AC charging function. This article dissects a typical case of "AC charging fails, DC charging works fine."

Fault Symptoms

Root Cause Analysis

1. AC charging station/gun fault — Station-side issue, verifiable by swapping stations (excluded in this case).
2. OBC low-voltage power supply fault — Abnormal OBC power supply.
3. Internal OBC fault — Internal component (e.g., thermistor, rectifier) damaged.
4. What does normal DC charging indicate — DC fast charging charges the traction battery directly without passing through the OBC rectifier. Therefore, when AC fails but DC works, the problem is concentrated in the AC charging path and the OBC itself.

Diagnostic Procedure

1. Read DTCs to define the fault: Connect the diagnostic scan tool. The OBC and VCU store P1A8998 "Thermistor failure" and P1C2C04 "OBC fault, emergency shutdown," both current faults.
2. Corroborate with live data: The VCU live data stream already detects the AC gun insertion signal, but after the VCU commands the BMS to drop high voltage, the traction battery still cannot be charged — the charging command chain is intact, and the problem lies at the OBC execution end.
3. Understand the OBC structure: The OBC on this vehicle has three layers — the top layer is the high-voltage distribution module (which distributes traction battery voltage to the DC charge port, motor, A/C, and PTC via two jumper boards, and connects the high-voltage interlock wiring harnesses and cover-open protection switches of each high-voltage connector), the middle layer is the water-cooled cooling channel, and the bottom layer is the OBC control module (rectifier, AC-DC conversion, and temperature management system). The OBC communicates data only via PCAN, with no separate temperature transmission channel.
4. Consult the OEM service manual: Possible causes for P1A8998 are "OBC low-voltage power supply fault or internal OBC fault." Measured OBC system communication is normal — low-voltage power supply fault is excluded, and an internal OBC fault is essentially confirmed.
5. Disassemble and locate the fault: Only the bottom control module section is disassembled for inspection. Inside, two thermistors are responsible for temperature control. Measure each one: one reads 842 kΩ, the other reads open circuit — the normal range should be 1.5~2.0 kΩ, so one thermistor is damaged.
6. Repair and verify: Replace the damaged thermistor, reassemble the OBC, refill and bleed the coolant, AC charging returns to normal and battery level rises; rescan the system with no DTCs, VCU live data is normal, and the fault is completely resolved.

Repair Plan

1. When the combined DTCs "Thermistor failure + OBC emergency shutdown" appear, first verify whether OBC communication is normal to quickly distinguish between a low-voltage power supply issue and an internal fault.
2. After confirming an internal fault, disassemble only the OBC bottom control module to avoid full-unit teardown, and focus on measuring the two thermistors.
3. After replacing the thermistor, reassemble per specification. Coolant must be refilled and bled to prevent air pockets in the cooling channels from causing heat dissipation failure.
4. After repair, perform a full charging verification with an AC station and confirm no DTCs remain in the system.

Key Parameter Quick Reference

Safety Warnings

• Before disassembling the OBC, the vehicle must be powered down and the traction battery service disconnect switch must be opened. Perform voltage verification before working.
• The OBC contains large capacitors. After power-down, wait for discharge to complete or discharge per the manual to prevent residual charge injury.
• After disassembling the cooling channels, coolant must be bled after refilling. Prevent coolant from splashing into high-voltage components during work.

Repair Insights

This case has two contrasts: First, "AC fails, DC works" is itself a clue — DC charging bypasses the OBC, so the fault scope can be directly narrowed to the OBC and the AC path; Second, measure two small resistors before replacing the whole unit — 842 kΩ and open circuit, compared to the normal range of 1.5~2.0 kΩ, a thermistor costing a few dozen yuan locks in the root cause. Determining whether the OBC is a "unit failure" or a "component failure" often comes down to just one resistance measurement.

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【This article is for reference only】