310S hybrid and EV systems practice questions.

Five free practice questions on hybrid and EV systems: block H of the Red Seal 310S Automotive Service Technician exam, about 9 of its 125 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Critical · Diagnoses electric vehicle (EV) systems

   In January, an EV owner in Québec reports that right after a DC fast charge to 100 % the car does not slow down when the accelerator is released the way it usually does. No DTCs are stored. What explains this?

   1. The friction brakes have failed
   2. The drive motor position sensor is faulty
   3. The brake pedal position sensor needs calibration
   4. Regen braking is limited when the pack is full and cold
   Show the answer

   D. Correct. Regeneration works by pushing current back into the pack. At 100 % state of charge there is nowhere for it to go, and a cold pack accepts charge even less readily, so the controller limits regen and the vehicle coasts. It is normal behaviour; the friction brakes take over.

   Why not the others

   A. The pedal-operated friction brakes are not involved in lift-off deceleration. A brake failure would show as long pedal travel and warning lamps, not as reduced coasting drag.

   B. A position (resolver) fault would cause harsh drive behaviour and DTCs at all times, not a temporary change only after a full charge.

   C. The pedal sensor blends friction and regen when the pedal is pressed. The customer's complaint is about lift-off behaviour with no pedal input.

2. Critical · Repairs electric vehicle (EV) systems

   After installing a replacement HV battery pack in an EV, the technician insulation-tests the pack output before reconnecting: HV+ to chassis reads 300 MΩ, HV− to chassis reads 0.4 MΩ. The minimum is 5 MΩ. What should be done?

   1. Replace the isolation monitor
   2. Do not energize; locate the fault on the negative side first
   3. Swap the HV+ and HV− cables at the pack
   4. Energize the system, since one side is well within specification
   Show the answer

   B. Correct. One side of the pack has a leak to chassis, and energizing it would set an isolation fault at best and expose people to shock at worst. Recheck the negative bus bar and cable routing for pinching, moisture, coolant or a damaged seal in the pack output connector, and re-test until both sides exceed the minimum.

   Why not the others

   A. The reading came from the insulation tester, not the vehicle's monitor. The fault is physical and on the negative side.

   C. Reversing polarity would damage the inverter and does nothing about a leak between the negative conductor and the chassis.

   D. Both sides must pass. A single fault to chassis makes the whole HV system dangerous the moment a second path appears, and the isolation monitor will refuse READY anyway.

3. Critical · Diagnoses high voltage batteries

   A hybrid's engine runs almost constantly, EV mode is unavailable, and the scan tool shows HV battery temperature at 52 °C with the battery cooling fan commanded at maximum. What should the technician check first?

   1. The inverter/motor loop coolant level
   2. The battery cooling air intake and filter
   3. The 12 V auxiliary battery condition
   4. The engine cooling system thermostat
   Show the answer

   B. Correct. Air-cooled hybrid packs draw cabin air through an intake grille near the rear seat; when the grille or filter is clogged with pet hair or blocked by cargo the fan runs flat out and the pack still overheats. The controller then limits charge and assist to protect the cells, so the engine runs constantly. Clean the intake and re-check temperatures.

   Why not the others

   A. The inverter/motor coolant loop does not cool an air-cooled hybrid pack. The data shows a hot battery, not a hot inverter.

   C. A weak 12 V battery causes start-up and module faults, not a pack that overheats under normal driving.

   D. The engine cooling system does not cool the HV battery, and the complaint is about the pack running hot, not the engine.

4. Procedural · Services high voltage batteries

   A technician is replacing one failed module in a hybrid HV battery pack. Which practice is correct?

   1. Install a new module at full charge so it carries the pack
   2. Use any module of the same physical size
   3. Match the module's voltage and condition to the remaining modules
   4. Replace all bus bars with copper wire jumpers
   Show the answer

   C. Correct. Modules in series must share state of charge and similar capacity; a mismatched module quickly drives the block voltages apart and brings the deterioration code back. Match voltage, capacity and age (or recondition the whole pack afterward), follow the manufacturer's bus bar removal sequence to divide the pack voltage, and torque the new hardware to specification.

   Why not the others

   A. A fully charged module in a partly charged string is a mismatch: it will be over-charged and the others under-charged, and the imbalance code will return.

   B. Physical fit does not mean electrical match. Capacity, chemistry and voltage must correspond to the rest of the pack.

   D. Bus bars are sized, plated and insulated for the current and the connections; wire jumpers overheat and loosen and are not an approved repair.

5. Critical · Diagnoses hybrid vehicle systems

   A technician removes the service plug from a hybrid, immediately opens the inverter cover and measures 180 V DC across the inverter's DC input terminals. What is the correct interpretation?

   1. The inverter capacitors are still charged; wait the specified time
   2. The service plug did not open the circuit, so a contactor is welded
   3. The meter is faulty, since voltage cannot be present with the plug out
   4. The DC-DC converter is boosting 12 V into the HV bus
   Show the answer

   A. Correct. Removing the service plug opens the pack, but the inverter's DC bus capacitors hold a lethal charge until they bleed down. Manufacturers specify a wait (typically 5–10 minutes) after which the terminals must be re-measured and read near 0 V before any HV work begins.

   Why not the others

   B. A welded contactor is a possible fault, but the first and far more common explanation is stored capacitor charge. Only if voltage remains after the specified wait does the technician suspect the pack is still connected.

   C. Voltage can absolutely be present with the plug out; that is why the wait time and the zero-voltage verification exist. Trust the meter and treat the terminals as live.

   D. The DC-DC converter steps HV down to charge the 12 V battery; it does not step 12 V up to the HV bus. Nothing on the 12 V side can produce 180 V here.

02Answer key

03What block H covers

This block covers hybrid and electric vehicles. It includes hybrid layouts, the 12 V battery and DC-DC converter, high-voltage batteries and state of health, contactors, pre-charge and interlock circuits, isolation faults and insulation testing, Level 2 and DC fast charging, electric A/C compressors and PTC heaters, cooling loops, and safe high-voltage service procedures.

The Red Seal Occupational Standard names this block “Diagnoses and repairs hybrid and electric vehicle (EV) systems”. It carries 7.2% of the exam, about 9 of the 125 questions, split across two tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Check the 12 V side first

A hybrid that will not go READY with a well-charged HV pack may have a flat 12 V battery, which powers the modules and contactor coils. In READY, the DC-DC converter should hold the 12 V system at 13.5 to 14.5 V.

Isolation faults and insulation tests

The isolation monitor sees the whole HV system as one. With it de-energized and verified, unplug and insulation-test components one at a time. A reading far below the manufacturer's minimum, such as 0.05 megohms against 10, condemns that part.

AC versus DC charging

AC charging at Level 1 or Level 2 goes through the onboard charger, while DC fast charging bypasses it and feeds the pack directly. A vehicle that fast-charges but will not charge on AC points to the onboard charger or its AC input.

POE oil in electric compressors

HV A/C compressors need the specified POE oil because it insulates the motor windings. PAG oil left in a shared recovery machine can lower insulation resistance and bring the isolation fault back, so use POE-dedicated equipment.

05The other blocks

The 310S exam has eight blocks. Each has its own page of free questions:

The 310S Automotive Service Technician practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.