Low-voltage support
Battery condition, module feeds, grounds, wake-up circuits and network health can prevent READY mode before the traction system is ever enabled.
Battery electric + hybrid field guide
ELECTRIC
VEHICLES.
Troubleshoot the supporting 12 V system, high-voltage enable sequence, battery management, charging, thermal control and propulsion network without crossing the boundary between diagnosis and unsafe energized work.
SYSTEM CLASSHVBEV / PHEV / HEV
This page is educational—not an authorization to work live. High-voltage service requires vehicle-specific training, rated PPE and test equipment, the exact OEM disable procedure, lockout/tagout, the specified wait time and verification of absence of voltage. The battery remains energized internally even when the vehicle is disabled.
EV DIAGNOSTIC LIBRARY
Choose a system path
Start with the system involved in the complaint, then move into a focused guide without losing the high-voltage safety boundary.
01 / EV LIBRARY
EV Foundations
Architecture, vehicle types, low-voltage dependencies and final repair verification.
EXPLORE 4 GUIDES →02 / EV LIBRARYBattery & High Voltage
Battery data, imbalance, isolation, interlocks, contactors and component relationships.
EXPLORE 6 GUIDES →03 / EV LIBRARYCharging Systems
AC charging, DC fast charging, communication, charge limits and slow-charge complaints.
EXPLORE 3 GUIDES →04 / EV LIBRARYDrive & Regeneration
Precharge, inverter and drive-unit diagnosis, plus regenerative-braking operation.
EXPLORE 3 GUIDES →05 / EV LIBRARYThermal Systems
Battery, power-electronics and cabin loops, including heat-pump diagnostics.
EXPLORE 2 GUIDES →06 / EV LIBRARYDiagnostic Resources
EV networks, scan data, scope tests, collision assessment and common mistakes.
EXPLORE 6 GUIDES →Know the boundary
SAFE
BEFORE SMART.
A silent vehicle may still be READY. Orange cables, battery enclosures and inverter terminals are never exploratory test points. Begin with service information, vehicle condition and a complete scan.
High-voltage disable
Use the model-specific service disconnect, secure the key, follow lockout/tagout and wait for capacitor discharge exactly as specified.
Absence of voltage
Use the OEM live-dead-live method and properly rated equipment at the specified measurement points. A scan PID is not proof of de-energization.
Damage, heat or off-gassing
Do not continue ordinary workshop diagnosis when the pack is damaged, hot, leaking, hissing, smoking or showing evidence of thermal distress.
Diagnostic order
READY MODE
IS A SEQUENCE.
A no-READY complaint is not automatically a failed traction battery. The vehicle must prove low-voltage stability, network communication, interlock continuity, isolation integrity and a successful precharge before closing its main contactors.
- Identify and stabilizeConfirm BEV, PHEV or HEV architecture; inspect for damage; support the 12 V system with the approved equipment.
- Complete vehicle scanRecord every module, DTC, freeze-frame and communication failure before clearing anything.
- Verify low voltageLoad-test the auxiliary battery, main grounds, fuses, wake-up feeds and DC-DC output strategy.
- Follow the enable chainCheck brake input, key authorization, charge-port state, crash status, interlock and module permissions.
- Evaluate isolation and prechargeUse scan data and the OEM flowchart to determine why contactors are inhibited or the DC link fails to rise correctly.
- Prove the repairRepeat READY, charging and controlled road-test conditions while monitoring temperatures, voltage, current and DTC status.
01 / NO READY
LOW VOLTAGE
LOW VOLTAGE
FIRST.
The auxiliary battery powers controllers and contactor coils. A weak 12 V supply can produce widespread communication, interlock and enable faults.
CHECK
Battery under real load
- State of charge and capacity
- Terminal and ground voltage drop
- Voltage during wake-up and READY request
COMPARE
Module permission PIDs
- Key authorization
- Brake and range status
- Crash and service-disconnect status
COMMON FAULTS
Network-wide symptoms
Multiple low-voltage or lost-communication codes often point to a shared supply problem rather than simultaneous module failure.
VERIFY
DC-DC converter behavior
Once READY is achieved, compare commanded and measured low-voltage charging with battery temperature and energy-management strategy.
02 / HV INTERLOCK
FIND THE OPEN.
FIND THE OPEN.
KEEP IT SAFE.
The high-voltage interlock loop monitors service disconnects, connectors and protected enclosures. An open loop normally prevents or drops contactor command.
BEGIN WITH DATA
Locate the reported segment
Read interlock status and related DTC subcodes. Use topology and connector views before touching the circuit.
INSPECT
Recent service points
Check charge-port, compressor, heater, inverter and battery connector locks only under the required safe state.
DO NOT
Never bypass the loop
Jumping an HVIL circuit defeats a safety function and can allow energization with a connector or enclosure unsecured.
PROVE
Repeat the complete enable
Confirm stable interlock status through key cycles, vibration-sensitive harness areas and the repaired connection.
03 / ISOLATION FAULT
LEAKAGE TO
LEAKAGE TO
CHASSIS.
The battery management system monitors electrical isolation between the floating high-voltage bus and vehicle chassis.
SCAN FIRST
Positive or negative side clues
Record isolation resistance, polarity hints, humidity and temperature conditions, and whether the fault appears during READY or charging.
COMMON AREAS
Moisture and coolant
- Electric A/C compressor
- PTC or coolant heater
- Charge inlet and onboard charger
- Battery or inverter coolant intrusion
CONTROLLED ISOLATION
Follow the OEM tree
Sectionalize only after safe disable and only at approved connectors. Never use an ordinary insulation tester on connected electronics.
STOP
Pack-internal evidence
Internal isolation failure, crash damage or fluid inside a sealed pack moves the job to qualified battery repair or replacement procedures.
04 / PRECHARGE + CONTACTORS
CONTROL THE
CONTROL THE
INRUSH.
Precharge raises inverter DC-link voltage through a resistor before the main contactor closes, limiting current and protecting contacts.
GRAPH TOGETHER
Pack voltage and DC link
During an approved READY request, the DC link should rise toward pack voltage within the specified time.
STATUS
Command versus feedback
Compare negative, precharge and positive contactor commands with feedback, current and welded-contactor checks.
FAILURE PATTERNS
Slow, absent or unexpected rise
- Open precharge path
- Failed resistor or contactor
- DC-link load or short
- Welded main contacts
CAUTION
No direct probing by default
Use scan data first. Direct HV measurement requires the OEM procedure, rated equipment and trained personnel.
05 / WILL NOT CHARGE
SEPARATE EVSE
SEPARATE EVSE
FROM VEHICLE.
Charging depends on the supply equipment, inlet, pilot/proximity communication, locks, onboard charger, battery permissions and thermal conditions.
EXTERNAL
Prove the charging source
Check indicator state, supply protection and operation with a known-compatible EVSE when the approved procedure permits.
VEHICLE INPUTS
Inlet and communication
- Connector seating and lock
- Pilot/proximity status
- Charge schedule and state of charge
- Temperature and isolation permissions
AC CHARGING
Onboard conversion
The onboard charger converts incoming AC to DC and coordinates voltage, current, temperature and battery limits.
DC FAST CHARGING
Different power path
If one charging mode works and another fails, use the architecture to isolate shared components from mode-specific communication and power paths.
06 / RANGE + PERFORMANCE
ENERGY AND
ENERGY AND
THERMAL DATA.
Reduced range or power can originate in the battery, temperature control, tires, brakes, driving conditions or an estimation issue.
BATTERY DATA
Compare, do not cherry-pick
- State of charge and usable energy
- Cell-voltage spread under load
- Pack current and temperature spread
- Charge and discharge power limits
THERMAL SYSTEM
Verify heat movement
Check pumps, valves, fans, refrigerant system, heaters and temperature sensors against commanded operation.
MECHANICAL LOAD
Look outside the battery
Tire pressure, alignment, brake drag, wheel bearings, accessory loads and ambient temperature materially change consumption.
ROAD TEST
Use repeatable conditions
Compare requested and actual torque, pack power, temperatures and energy use on a controlled route—not from one dashboard estimate.
Evidence set
CAPTURE
THE EVENT.
- Complete pre-scan with module reachability and DTC subcodes
- Auxiliary-battery voltage through wake-up and READY request
- Pack voltage, DC-link voltage and contactor command/feedback
- Isolation resistance and the exact operating state when it changes
- Cell-voltage and temperature spread under a controlled load
- Charging permissions, pilot/proximity state and requested/actual current
- Coolant temperatures, pump/valve command and thermal derating reason