Charging Fault Diagnosis

The customer says the car will not charge — or it starts charging and stops. Charging faults are one of the most common EV complaints, and they involve multiple systems that all have to agree before current flows. Understanding the charging handshake and each component's role is key to efficient diagnosis.

Before any current flows, the vehicle and the charging station communicate. On Level 2 AC charging, the EVSE sends a pilot signal to the vehicle indicating available current. The vehicle responds by closing its internal contactors and requesting current. On DC fast charging, the communication is more complex — the station and vehicle negotiate voltage and current limits through a digital protocol (CCS uses PLC communication). If any part of this handshake fails, charging does not start. The charge port indicator light on the vehicle often tells you where in the process the failure occurred — check manufacturer documentation for indicator color and flash pattern meanings.

Before diagnosing the vehicle, try a different charging station. EVSE units fail, trip breakers, lose network connectivity, and have their own internal faults. A vehicle that will not charge at one station but charges fine at another has a station problem, not a vehicle problem. Try both a Level 2 station and DC fast charging if available. If the vehicle fails on all stations, the problem is in the vehicle.

Look at the charge port physically. Check for bent or corroded pins, debris in the port, water intrusion, or a damaged port door that does not allow the connector to seat fully. The locking mechanism must engage — most vehicles will not begin charging until the connector is locked in place. A stuck or broken lock actuator prevents charging. Check the charge port inlet temperature sensor — if it reads high due to a fault, the system limits or prevents charging as a safety measure.

Charging involves more modules than you might expect. The onboard charger converts AC to DC for the battery on Level 2 charging. The BMS monitors cell voltages and temperatures and can stop charging if any parameter is out of range. The HV battery contactors must close to allow current flow. The thermal management system must be operating to manage battery heat during charging. The ground fault detection system monitors HV insulation — any insulation breakdown stops charging immediately as a safety measure. Scan the EV control module, BMS, onboard charger module, and body control module. A DTC in any of these can prevent charging.

The HV system continuously monitors insulation resistance between the HV circuits and the vehicle chassis. This is called ground fault detection or insulation monitoring. If the insulation resistance drops below the threshold — from damaged cable insulation, moisture intrusion, a failing component, or coolant leaking onto HV components — the system sets a ground fault code and disables charging and sometimes driving. Ground fault codes require careful inspection of all HV components, cables, and connectors for physical damage, moisture, or contamination. This is a safety system — it is telling you that touching the vehicle chassis could put you in the HV circuit. Take it seriously.

After any charging system repair, clear codes and perform a full charge cycle. Monitor the charging process with the scan tool — watch for current flow, voltage levels, temperature management, and any new codes. Verify that charging completes to the target level without interruption. Test on both Level 2 and DC fast charge if the vehicle supports both.