HV Battery Precharge Circuit Takes Too Long

Common Symptoms

• Vehicle will not start or move, dash shows 'Service High Voltage Charging System' or propulsion-loss warning
• Red or amber warning lights illuminate on the instrument cluster immediately after pressing the power button
• Car appears to power up normally but then shuts back down before reaching READY state
• Chevrolet Bolt EV displays 'Propulsion Power Is Reduced' or 'Service EV System' message
• Climate control and 12V accessories may work briefly but drive is unavailable
• P0C78 logged as a current or stored DTC alongside one or more U3xxx network faults
• Repeated press of the power button produces the same failed startup sequence every time

Probable Causes (Ranked by Likelihood)

• Blown or degraded fuse inside the Manual Service Disconnect (MSD) pack, preventing precharge current from flowing through the precharge resistor circuit Very Likely
• Failed precharge relay inside the Battery Energy Control Module circuit, stuck open so high-voltage never ramps to inverter bus voltage within the timeout window Very Likely
• Precharge resistor open-circuit or high-resistance fault, slowing the voltage rise on the DC bus below the BECM threshold timer Likely
• High-voltage contactor (positive main or precharge path) corroded or failing to close fully, causing slow or incomplete bus charge Likely
• Significant high-voltage bus leak to chassis (isolation fault) causing the precharge circuit to bleed off energy faster than it builds, triggering the timeout Possible
• Degraded LG cell module capacity or extreme low state-of-charge preventing adequate source voltage for the precharge ramp Possible
• BECM software fault or corrupt calibration causing an incorrect timeout threshold, especially on pre-updated 2017-2019 model years Less Likely

Step-by-Step Diagnostic Procedure

1. Pull the DTC with GM Tech 2 / Global Diagnostic System (GDS2) or a compatible third-party tool such as Autel MaxiSys or Foxwell NT530 with GM software. Confirm P0C78 is current, not just stored history. Note every co-DTC, especially U3xxx codes, because network faults can cause a false precharge timeout.

2. Locate the Manual Service Disconnect (MSD) on the driver-side rear of the battery pack, accessible through the rear seat cargo area. With the vehicle powered OFF, carefully remove the MSD handle. Visually inspect the internal fuse element through the clear window. A blown fuse is visible as a broken wire inside the glass. This is the most common single cause documented in GM bulletin 18-NA-225.

3. If you have a multimeter, set it to DC voltage and, with the MSD removed for safety, check continuity across the MSD fuse terminals using the resistance (ohms) setting. You should read near zero ohms. An open reading confirms the fuse is blown. Replace only with the exact GM-specified fuse (150A, part number varies by model year, confirm via GM parts catalog before ordering).

4. After reinstalling a new MSD fuse, attempt a normal power-on cycle. If the vehicle reaches READY state and P0C78 does not immediately return, the MSD fuse was the root cause. Clear the DTC with your scan tool and verify no reoccurrence over several drive cycles.

5. If P0C78 returns after a new MSD fuse, use GDS2 to monitor the High Voltage Bus Voltage parameter in real time during a startup attempt. You want to see the bus voltage climbing steadily from pack voltage toward a value matching the traction battery open-circuit voltage within roughly 3-5 seconds. A flat or very slow rise points to a failed precharge relay or open precharge resistor, both inside the battery pack assembly, which require dealer or advanced EV shop service.

6. Check for any high-voltage isolation fault codes (P0AA6 or similar) alongside P0C78. A ground fault on the HV bus will cause the precharge circuit to fight an unintended load, making the timeout trigger even on hardware that is otherwise intact. An isolation fault requires GDS2 live data and often a megohmmeter to locate.

7. Inspect the 12V auxiliary battery voltage. The precharge contactor sequence is controlled by the BECM using 12V power. A weak 12V battery (below 11.5V under load) can cause contactor timing faults that mimic a precharge failure. Measure 12V battery voltage at the jump-start terminals under the hood with a multimeter. Charge or replace the 12V battery if it reads below 12.4V at rest.

8. If all accessible checks pass and P0C78 still sets, the fault is inside the sealed battery pack, most likely a failed contactor or precharge relay. This requires battery pack disassembly, high-voltage training, and GDS2-level diagnostics. At this point, schedule service at a Chevrolet dealer or certified EV repair facility.

Frequently Asked Questions