What it means
The hybrid/EV battery system precharge time is longer than the calibrated limit. The code indicates an abnormal precharge-time result; its cause depends on the vehicle.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
During startup, the high-voltage battery connects to the inverter through a precharge path that gradually brings the inverter capacitors toward pack voltage before the main contactors close. The control module monitors how long this voltage buildup takes; P0C78 sets when precharge exceeds the vehicle’s calibrated time limit, which can result from resistance in the path, weak contactor operation, or a battery condition affecting the process.
Common causes
Most likely first.
- Increased resistance in path
- Contactor coil weakening
- Pack cell imbalance approaching service threshold
Symptoms
- EV/hybrid system warning light on
- Reduced power output or limp mode
- Charging may fail or be slowed
- Reduced electric range
How to diagnose it
- Read freeze-frame data and hybrid-system codes to see when precharge was slow and whether battery or contactor faults were recorded at the same time.
- Compare live precharge voltage and timing data with the manufacturer’s specification to confirm that voltage rises too slowly.
- With the vehicle safely powered down according to its service information, inspect accessible precharge-path connectors and wiring for damage, looseness, or corrosion.
- Have the precharge circuit and contactor operation checked using the vehicle’s specified high-voltage service procedure.
- Review battery-management data for cell imbalance or other pack faults that could affect precharge before replacing components.
Common fixes
- Repair a loose, damaged, or corroded precharge-path connection.
- Repair wiring faults in the precharge circuit.
- Replace a precharge contactor or related component confirmed to be weak or faulty.
- Address a battery cell imbalance or pack fault identified by the battery-management system.
Can I keep driving?
Avoid relying on the vehicle for routine driving if it has reduced power, limp mode, or unreliable startup; it may fail to enter drive or lose propulsion. Do not open or probe high-voltage components yourself.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.