Changan Plug-in Hybrid VCU Diagnosis and Removal/Installation Guide
Fault Background
The vehicle control unit (VCU) of the Changan plug-in hybrid (777 manual) is located in the front compartment. It processes input signals by functional module and outputs to each subsystem: torque distribution module, start/stop module, energy recovery module, temperature control module, fault diagnosis module. The low-voltage connector is 72-pin, with key pins including: VCU power self-hold signal (digital output 0.6A), Auto mode indicator (50mA), 5V sensor supply, BATTERY switch control output (0.3A nominal/0.5A peak), pin 46 charge wake-up signal Chgwakep, pin 50 crash signal Crash Sig (PWM input, normally high; on fault, port pulled low for 200ms). The largest family of DTCs is "message counter/CRC/timeout" for each controller (IPU/BCU/DCDC/EMS/TCU/ACU/EPBI/SRS/TMS/GW/OBC, over a dozen IDs), all sharing the same troubleshooting template; there are also specific codes for crash, insulation, interlock, accelerator pedal, auxiliary battery, vacuum pump, cruise switch, etc. This article is organized as a diagnosis and removal/installation guide.
Message Timeout Family · Unified Troubleshooting Template
Template four steps (applicable to IPU ID153/173/2A3/303, BCU ID185/215/235/285/325, DCDC ID2C9/419, EMS ID205/255/26A, TCU ID239/339, ACU ID15A/25A, EPBI ID208/218/228/258/278/256, SRS ID050, TMS ID324/32C, OBC ID319, GW forwarding types):
- Check whether the controller power supply and ground are normal → if abnormal, troubleshoot the power supply cause;
- Check whether the controller CAN line to the VCU vehicle CAN terminal is shorted to ground, shorted to power, or open → if abnormal, replace the wiring harness or connector;
- Replace with a VCU from the same batch / replace the forwarded controller, and see if the fault is resolved;
- If still not resolved → contact the corresponding controller/coupling mechanism/battery pack manufacturer to confirm internal component status to diagnose lost frames.
Counter/CRC type (transmitted message checksum and counter faults) do not test the wiring harness; directly contact the corresponding manufacturer to diagnose internal component status.
BCU ID185 special case: First check the continuity/short-to-ground/backed-out terminal of the wiring harness between the VCU's BMS enable pin and the battery pack connector → replace connector or harness; check whether the corresponding VCU pin outputs high level → otherwise replace with a same-batch VCU.
Specific DTCs
| DTC | Meaning | Troubleshooting Points |
|---|---|---|
| P1816 | Crash fault | Airbag connector and VCU pin 50 crash signal line continuity → pin 50 short to power/ground → replace airbag → replace with same-batch VCU |
| P1817 | Insulation fault | Power down the vehicle, disconnect the harness between battery and charger, use insulation tester to measure main positive-to-ground/main negative-to-ground → if abnormal, check insulation of external high-voltage components of battery pack → if normal, contact battery pack manufacturer |
| P1818 | High-voltage interlock fault | Compare battery pack total voltage with IPU voltage via live data → use multimeter to check battery output continuity at IPU connector → check battery pack → disconnect service disconnect switch, lift vehicle, remove high-voltage busbar protective cover, check connections → IPU high-voltage connector → battery pack/IPU low-voltage connector terminal damage or deformation |
| P181A | Accelerator pedal 1 out of range | Check connector secure, no backed-out pins → use multimeter to check continuity between accelerator pedal and corresponding VCU pin → replace accelerator pedal → replace with same-batch VCU (accelerator pedal 2 out of range/offset refer to this) |
| P181D | Brake switch rationality | Same path as accelerator pedal: connector → continuity → replace brake pedal → replace with same-batch VCU |
| P181E | Auxiliary battery voltage low level 1 | Auxiliary battery voltage (approx. 12V when not powered) → check continuity between DCDC positive output line and auxiliary battery → when ON gear READY light flashes, voltage clearly increases by more than 0.5V → wait 10 minutes in ON gear → fuse on 12V battery positive terminal blown → replace with same-batch VCU (level 2 fault refer to this) |
| P1823 | Electronic vacuum pump fault | Read vacuum pressure value via live data, press brake pedal deeply multiple times until vacuum pump operates: if change is slow and fault reappears after one minute → check vacuum pump and lines for leaks → replace vacuum pump or lines → replace with same-batch VCU |
| P1824 | VCU CAN1 hardware fault | VCU power and ground → CAN line short to ground/power/open → replace harness → replace with same-batch VCU (CAN2 refer to this) |
| P187B | OBC recoverable fault mode | Charging cable plug end voltage below 150V → check power supply; charger temperature above 75°C → check temperature; charging station card operation timeout >3min → re-swipe card to charge |
| P187D | Cruise switch fault | Cruise switch connector and VCU connector secure → continuity between cruise switch and VCU brake signal corresponding pin → resistance value into VCU normal after pressing → replace cruise switch → replace with same-batch VCU |
| P1878 | ECU fault level 2/3 limp, level 4 | Refer to corresponding DTC in the ECU Service Manual |
| P1885 | TMS ID324 message timeout | Forwarded controller power and ground → its CAN line → replace forwarded controller → GW controller power and ground → GW CAN line → contact GW manufacturer |
| — | TCS/ABS/EBD/EEPB failure | Perform ESC system test; TCU limp → TCU system test |
Inspection and Adjustment / Removal and Installation
Inspection: Check connector for cracks; replace if cracked. Check VCU assembly for deformation or cracks; replace if cracked. Retighten mounting bolts/nuts to specified torque if necessary.
Removal/Installation precautions: ①Before removal/installation, cut off high voltage (key OFF + disconnect service disconnect switch); ②Must disconnect the lead-acid battery negative terminal to ensure VCU low-voltage power is off; ③Tighten bolts to specified torque to ensure assembly reliability.
Removal/Installation: Pull down the low-voltage connector latch and disconnect the low-voltage connector → use torque wrench to remove fasteners and detach VCU from body. VCU has 3 mounting holes: 1 hex flange bolt (09103-06011-S353) + 2 hex flange nuts (9159-06005-S112), torque 13±1N·m, mounted on the VCU assembly bracket below the right side of the center console; installation is reverse of removal.
Safety Warnings
- This article is for technical reference. VCU repair involves high voltage and vehicle control; must be performed by qualified professional technicians (electrician certificate + company training authorization) per OEM specifications.
- Double power-off: key OFF + disconnect service disconnect switch, then disconnect lead-acid battery negative terminal; all are mandatory; retighten to 13±1N·m torque during reassembly.
- Crash signal (pin 50) is linked to airbag; never measure airbag circuit with power applied during troubleshooting; follow manual steps to disconnect and test.
- The message timeout family involves multiple controller manufacturer parts; first check power supply and CAN harness, then replace VCU, finally contact manufacturer—order must not be reversed.
Repair Insights
The most valuable lesson from this VCU code table is the "message timeout family" troubleshooting template: over a dozen controllers, dozens of IDs (IPU 153/173/2A3/303, BCU 185/215/235/285/325, DCDC 2C9/419, EMS 205, TCU 239, ACU 15A, EPBI 208, SRS 050……) all converge to four steps—power/ground → CAN line → replace VCU/forwarded controller → contact manufacturer. Remember this template; you won't need to look up the manual for any "ID message timeout/counter/CRC" code. The three behavioral codes are also interesting: P187B's 150V/75°C/3min lists all three recoverable causes—"insufficient supply, charger overheating, charging station card swipe timeout"—at once; don't rush to replace parts. P181E's "0.5V or more voltage increase" is the fastest way to use a multimeter to determine whether the DCDC is charging the 12V system. P1823's "press brake to hear vacuum pump + fault reappears after one minute" catches intermittent vacuum pump failure accurately. Finally, the discipline of crash code P1816: start with connector continuity, only replace the same-batch VCU after replacing the airbag and the fault persists—for safety-related components, the replacement order itself is a safety procedure.
【This article is for reference only】
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⚠️ High-voltage repairs require professional certification; this article is for technical reference only — always follow the OEM service manual.