Battery Charger Precharge Sequence Timeout Fault

Common Symptoms

• MIL (check engine light) illuminates on the instrument cluster
• Vehicle displays a propulsion power reduced or loss of propulsion warning
• Car fails to enter READY state when you press the brake and push the power button
• High-voltage contactors click but the vehicle does not power up fully
• 12V battery may drain faster than normal because the BECM keeps cycling
• Charging via Level 1 or Level 2 stops mid-session or fails to start
• GDS2 or a compatible scan tool shows freeze-frame data with pack voltage well below normal precharge target

Probable Causes (Ranked by Likelihood)

• Failing or degraded high-voltage precharge resistor inside the BECM contactor assembly not building bus voltage fast enough Very Likely
• Weak or corroded manual service disconnect (MSD) connection causing excess resistance in the precharge circuit path Very Likely
• One or more HV contactors sticking, not fully closing during the precharge sequence Likely
• BECM software fault or corrupted calibration requiring reflash, particularly on 2017-2018 build dates before mid-cycle software updates Likely
• High internal resistance in the LG battery pack cells due to age, cold soak below 10 degrees F, or early cell degradation covered under NHTSA 21V-560 recall Possible
• Corroded or loose high-voltage orange cable connectors between the pack and the drive unit increasing precharge load Possible
• Failed OBCM pulling excess current during the precharge window and causing a timeout Less Likely

Step-by-Step Diagnostic Procedure

1. Connect GDS2 or a GM-capable scan tool (Autel MaxiSys, Foxwell NT530 with GM software, or BlueDriver with the Bolt-specific code database) and read all stored DTCs. Note whether P0C78 or any U3xxx network codes appear alongside P0D26 because co-codes change the likely root cause.

2. Check the freeze-frame data for the HV bus voltage at time of fault. If it is below roughly 280V at the moment the precharge timeout fired, the resistor or MSD is the first suspect. If bus voltage looks normal, focus on the contactor and BECM software.

3. With the vehicle powered down and the 12V negative cable disconnected, locate the manual service disconnect (orange T-handle under the rear seat). Remove it, inspect the contacts and the plug body for corrosion, heat discoloration, or melted plastic. Reseating the MSD firmly has cleared this code on some early 2017-2018 vehicles.

4. Inspect the 12V auxiliary battery voltage with a multimeter. It should be 12.4-12.8V with the car off and 13.5-14.5V with the car in READY. A weak 12V battery can prevent the BECM from completing contactor sequencing correctly. Replace if below 12.0V at rest.

5. With the MSD removed and HV circuits de-energized, visually inspect the orange HV cable connectors at the battery pack and drive unit for green or white corrosion, bent pins, or loose locking tabs. Do not probe HV conductors directly without proper PPE and HV training. If you see damage, stop and call a qualified EV technician.

6. Clear the DTC with your scan tool, reinstall the MSD, and attempt to bring the vehicle to READY while monitoring live data. Watch the HV bus voltage ramp during precharge. On a healthy system it should reach target voltage within a few seconds. A slow or stalled ramp confirms a resistor or contactor issue.

7. If the code returns immediately and GDS2 confirms the precharge resistor circuit, the repair requires BECM contactor assembly replacement. This is a dealer or advanced independent EV shop job. Do not attempt to bypass the precharge circuit under any circumstances.

8. Check GM TechConnect or ask your dealer to verify whether an open BECM software calibration update applies to your VIN before authorizing parts replacement. GM issued revised BECM calibrations for 2017-2019 Bolt EVs that adjusted precharge timeout thresholds.

Frequently Asked Questions