EV Diagnostics, Service & HV Safety Interview Questions & Answers

eMobility Academy — EV Diagnostics, Service & HV Safety interview questions

1. Q1.What voltage is considered high voltage in an EV?

   Fresher

   Under ISO 6469-3, which vehicle safety rules follow, voltage class B (high voltage) is anything above 60 V DC up to 1500 V DC, or above 30 V AC rms up to 1000 V AC rms. Below those levels is class A. So a 400 V car pack is clearly HV, and a 72 V scooter pack (about 84 V fully charged) is also class B, while a 48 V pack (up to about 54 to 58 V) is below 60 V DC. Even class A packs can deliver very large short-circuit currents that cause burns, arc flash and fires, so tools must be insulated and jewellery removed. HV components in cars are marked with orange cables and warning labels.

2. Q2.What PPE is required for working on an EV high-voltage system?

   Fresher

   The core PPE is insulated gloves rated for the system voltage, typically Class 0 (maximum use voltage 1000 V AC and 1500 V DC under IEC 60903), worn with leather over-gloves to protect against cuts. Before each use, gloves are inspected and air-tested for pinholes and checked for their test date. Add safety glasses or a face shield for arc flash, non-conductive safety footwear, flame-resistant clothing without metal parts, and no watches, rings or chains. Use insulated tools rated to 1000 V (IEC 60900), an insulated mat, barriers and HV warning signs around the vehicle, and an insulated rescue hook nearby. A trained second person should be present for live HV work.

3. Q3.Describe the procedure to safely de-energise an EV before working on HV components.

   Fresher

   Follow the OEM service manual, but the standard steps are: switch off the vehicle and keep the key or smart key away from it; disconnect the 12 V battery negative so contactors cannot close; remove the manual service disconnect or HV fuse if fitted, and keep it in your pocket under lockout and tag-out; wait the time specified by the OEM, often 5 to 10 minutes, for the DC-link capacitors to discharge; then verify zero voltage with a CAT III or CAT IV 1000 V rated meter using the live-dead-live method at the specified test points, including between each HV terminal and chassis. Only after confirming the absence of voltage do you start work, and you still treat the battery itself as always live.

4. Q4.What is the live-dead-live test?

   Fresher

   Live-dead-live proves your meter is working before trusting a zero reading. First, test the meter on a known live source of similar voltage, such as a proving unit or the 12 V battery for function, to confirm it reads correctly. Second, measure the circuit you are working on and confirm it reads zero, or below the safe threshold, between all relevant points: HV+ to HV−, HV+ to chassis and HV− to chassis. Third, test the meter again on the known live source to confirm it did not fail during the measurement. If a meter fails silently, for example from a blown fuse or broken lead, it would read zero on a live circuit, and live-dead-live catches that.

5. Q5.What is an insulation resistance test and how is it done?

   Fresher

   An insulation resistance test checks that HV conductors are properly insulated from the chassis. With the system de-energised and the component isolated, an insulation tester (megger) applies a DC test voltage, typically 500 V or 1000 V as specified by the OEM, between each HV conductor and chassis ground, and reads resistance in megaohms. Healthy components usually read in the hundreds of megaohms or more. The minimum vehicle requirement under ISO 6469-3 and UNECE R100 is 100 Ω/V for DC circuits, about 40 kΩ for a 400 V system, but service limits are much higher. Low readings point to moisture, damaged cable insulation or internal component faults. Never apply a megger to low-voltage electronics, which it can damage.

6. Q6.What is a DTC and how is it structured?

   Fresher

   A diagnostic trouble code is a code an ECU stores when it detects a fault. The common format has a letter and four characters: P (powertrain), C (chassis), B (body) and U (network or communication). The second character shows whether it is generic (0) or manufacturer-specific (1, and some ranges of 2 and 3). For example, a U-code often indicates lost communication with a module, while P0A-series codes cover hybrid and EV powertrain faults such as isolation or battery faults. UDS-based systems store DTCs as 3-byte values with a status byte that shows whether the fault is active, pending or confirmed, plus freeze frame or snapshot data captured at the moment of the fault, which is often the most useful information.

7. Q7.A customer's e-scooter does not start. How would you approach it?

   Fresher

   Work from simple to complex. Check the display and warnings: is anything powering up, and is there an error code? Check battery SoC and charge the scooter if low. Confirm the side stand switch, brake switch and kill switch are in the correct state, since many scooters will not enable drive unless the brake is held or the stand is up. Check the key or keyless authentication and the 12 V auxiliary supply. Read faults with the OEM's diagnostic app or tool. If the battery shows healthy but there is no drive, check connectors between battery, controller and motor for burn marks or loose pins, check the throttle signal (typically about 0.8 V to 4.2 V across its travel), and check the Hall sensor connector. Document findings before replacing parts.

8. Q8.Why should you never use a regular multimeter for HV measurements?

   Fresher

   Meters carry a measurement category (CAT) rating and a voltage rating that show the transient overvoltage they can survive safely. A cheap unrated meter may arc internally when exposed to a high-energy HV circuit or a transient, causing an arc flash in your hand. For EV work you need at least a CAT III 1000 V meter, CAT IV preferred near the battery, with HV-rated test leads that have shrouded tips and finger guards and fused current inputs. Check the leads for damage before each use. Also never measure voltage with the leads in the current jacks, which would short the circuit. This question tests safety culture more than meter knowledge.

9. Q9.What should you do if an EV battery is damaged after an accident or shows signs of thermal runaway?

   Fresher

   Treat it as an emergency. Keep people away, do not touch the vehicle if HV cables or the pack are exposed, and call the fire service if there is smoke, hissing, popping or a sweet chemical smell, which indicates venting cells. Lithium-ion fires can reignite hours or even days later, so firefighters use large quantities of water to cool the pack; small extinguishers are not enough. A damaged pack that is not burning should be moved, by trained staff using the OEM's emergency response guide, to an isolated quarantine area outdoors, away from buildings and other vehicles, and monitored for temperature rise. Never charge a damaged pack. Follow the OEM's procedure for transport as a damaged battery.

10. Q10.A car shows an isolation fault DTC. How do you find the faulty component?

    Experienced

    Read the DTC and freeze-frame data: the isolation resistance value, whether the fault is on HV+ or HV−, and whether it occurs while driving, charging or with the AC on. That narrows it down, since a fault only during AC use points to the compressor or PTC heater, and one only while charging points to the OBC or inlet. Then de-energise safely and isolate subsystems: disconnect HV connectors one at a time and use an insulation tester at the OEM-specified voltage on each component and cable to chassis. Moisture in connectors, chafed cables near suspension or sharp edges, coolant leaks into electronics, and failed compressor windings are common causes. Check connectors for corrosion and water ingress, then repair and verify that the isolation reading in live data has recovered.

11. Q11.How do you diagnose a pre-charge failure?

    Experienced

    A pre-charge failure means the DC bus did not reach the expected percentage of pack voltage within the expected time. If bus voltage rises too slowly or not at all, suspect a short or heavy load on the HV bus (a failed inverter, DC-DC or compressor, or a shorted DC-link capacitor), an open pre-charge resistor or a failed pre-charge contactor, or a faulty bus voltage sensing circuit. If it rises too fast, suspect a disconnected load or missing capacitor. Diagnostics: check live data for pack and bus voltage during the attempt, check pre-charge resistor resistance and contactor coil operation, then disconnect HV loads one at a time and retry to find the one that drags the bus down, checking each with an insulation tester and for shorts.

12. Q12.How do you detect a welded contactor?

    Experienced

    BMS logic usually detects it: after commanding a contactor open, the BMS measures voltage across it or on the bus. If the bus voltage stays at pack voltage when only one contactor should be closed, or does not decay after both open, the contactor is flagged as welded and a DTC is set, often preventing the next HV enable. In the workshop, confirm with live data, then with the HV system de-energised and the pack isolated, measure continuity across the contactor's main terminals with the coil de-energised. Following OEM procedure, replace the contactor, inspect for the cause (for example pre-charge failures, high inrush, or opening under load), and verify that the BMS clears the fault after a correct open and close test.

13. Q13.A customer complains that AC charging stops after a few minutes. How do you troubleshoot?

    Experienced

    Collect history: which charger, which socket, time of day, error codes on the car and on the charger, and whether DC charging works. Check the supply: socket and plug for heat damage or loose terminals, voltage under load, and whether the supply circuit's MCB or RCD trips, since a portable charger on a weak or overloaded domestic circuit is common. Check the EVSE: control pilot state and duty cycle with an EVSE adapter or tester, proximity resistor value, and any ground fault indication. Check the vehicle: OBC temperature and fault codes, inlet temperature sensor (many inlets derate or stop on high pin temperature), and the 12 V system. Reproduce with a known-good EVSE to split the fault between vehicle and charger.

14. Q14.How do you read and interpret freeze-frame data?

    Experienced

    Freeze-frame (or snapshot) data records key parameters at the moment a DTC was set: SoC, pack voltage and current, cell min and max voltages, temperatures, vehicle speed, motor speed and torque request, 12 V voltage, odometer and timestamp. Read it before clearing codes, because clearing usually erases it. Interpretation means looking for the condition that triggered the fault: an undervoltage DTC with high current and low temperature suggests high cell resistance in cold conditions; a communication DTC with low 12 V voltage suggests a supply problem rather than a module failure. Compare multiple occurrences and the occurrence counter to find patterns. Freeze-frame data and the DTC status byte together tell you whether a fault is intermittent or permanent.

15. Q15.How do you diagnose an intermittent CAN communication fault?

    Experienced

    Identify which modules lose communication from the U-codes across all ECUs; a common subset points to a shared branch or connector. Measure termination resistance (about 60 Ω between CAN_H and CAN_L with power off) and check for shorts to ground or battery. With power on, check CAN_H and CAN_L voltages (about 2.5 V recessive, around 3.5 V and 1.5 V dominant) and look at the signal on an oscilloscope for noise, ringing or a stuck line. Then wiggle-test connectors and harness sections near moving parts, water entry points and HV cables while monitoring the bus or error frames with a CAN analyser. Check module ground and supply voltage, since a sagging 12 V supply can reset a module and look like a network fault.

16. Q16.How do you check the health of an EV battery during a pre-owned vehicle inspection?

    Experienced

    Use the OEM's diagnostic tool or a reliable scan tool to read SoH as reported by the BMS, cell voltage spread at rest and under load, the temperature spread, stored and historical DTCs, the number of fast-charge sessions and total energy throughput if available, and any isolation faults. A large cell voltage spread at high or low SoC indicates imbalance or a weak group. Then do a controlled test: charge to full, drive a known route and calculate Wh/km and usable kWh against the original spec, or use a capacity test where tools allow. Check the pack enclosure for impact damage, corrosion and signs of water ingress, and check the warranty status and service records.

17. Q17.What would you include in a high-voltage workshop safety setup?

    Experienced

    A dedicated HV work bay with barriers, floor marking and warning signs; trained and authorised staff with documented competency levels (aware, competent, expert); PPE with inspection and test records for gloves; insulated tools and CAT III or CAT IV meters, insulation testers and proving units; lockout and tag-out kits for service disconnects; an insulated rescue hook and first-aid with CPR-trained staff; a quarantine area outdoors for damaged batteries, plus a thermal camera; appropriate fire response equipment and a plan agreed with the local fire service; a battery lifting table and storage racks; and standard operating procedures with a permit-to-work for HV jobs. Periodic audits and refresher training keep the culture alive.

18. Q18.Motor controller shows a Hall sensor fault on an e-rickshaw. How do you verify it?

    Experienced

    With the vehicle on stands and drive disabled, back-probe the Hall sensor connector: check the supply (often 5 V) and ground, then rotate the wheel slowly by hand and observe each of the three signals with a multimeter or, better, an oscilloscope. Each output should toggle between low and high, and the three together should step through six valid states in sequence; a state of all-low or all-high means a dead sensor or wiring fault. Inspect the cable where it enters the motor, since hub-motor Hall cables commonly break or get chafed and water often enters there. Replace the sensor board or harness as required, then re-run the controller's Hall learning or auto-identification routine if it has one.

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