EV Specifics
Electric vehicles flip the PPI priority list.
By Anthony LimpertASE Master Automobile Technician- EV battery SoH is the equivalent of engine compression on an ICE car. A 5-year-old Bolt with 92% SoH is worth much more than one with 78% — and can predict future range. Use OBD Fusion, Leaf Spy, or OEM tools.
- Tesla-specific: pre-2018 Model S/X MCU eMMC memory wear is a known $2-3K repair. Drive units (motor + inverter) have specific failure modes. 12V battery failure will strand the car. Check service history for these.
Electric vehicles flip the PPI priority list: there is no oil to smell or compression to test, and the single most valuable component is the traction battery. Battery health data plus a handful of platform-specific known issues make up the core of a competent EV inspection.
Battery Health and the Tesla Checklist
State of Health, SoH, is the EV equivalent of a compression test on a gas engine: it is the number that tells you how much life the most expensive component has left. Pull it with an OEM-level scan tool or a proven aftermarket EV diagnostic tool such as OBD Fusion or Leaf Spy. Degradation hits both range and resale directly; a five-year-old Chevy Bolt at 92 percent SoH is worth meaningfully more than the same car at 78 percent, and the SoH trend also predicts the range the buyer will actually live with. Do not accept a voltage reading, a charging demonstration, or the dashboard range estimate as a substitute for a real SoH measurement.
On used Teslas, battery health is only the start of the list. Pre-2018 Model S and X cars have a known MCU failure mode: the main computer unit's eMMC flash memory wears out from constant logging, and the repair runs 2,000 to 3,000 dollars. Drive units, the motor and inverter assemblies, have their own documented failure modes, so listen for drive unit whine and check the service history for prior replacements. Finally, check the age of the 12-volt battery; when it dies it strands the car regardless of how much charge the main pack holds. Service records are your best friend on these cars, because the known issues are well enough documented that prior repairs are usually traceable.
| Item | Guideline | What it means |
|---|---|---|
| State of Health (SoH) | Pulled with an OEM-level or proven aftermarket tool (OBD Fusion, Leaf Spy class) | The compression test of EVs — dash range estimates and charging demos are not substitutes |
| SoH around 90%+ on an older pack | Strong for age | Range and resale advantage — a 5-year-old Bolt at 92% is worth meaningfully more than one at 78% |
| SoH below about 80% | Meaningful degradation | Negotiate on it, and check whether pack warranty coverage still applies to the VIN |
| Pre-2018 Tesla S/X MCU | eMMC flash wears out from constant logging | Known failure, $2,000-3,000 repair |
| Drive units | Whine on the drive, prior replacements in service history | Documented failure modes — history is usually traceable |
| 12V battery age | Aged 12V is a near-term line item | When it dies it strands the car regardless of main pack charge |
SoH bands are rough guides — degradation varies with chemistry, climate, and charging habits. Anchor the finding to scanned SoH plus the full-charge displayed range against the original EPA rating.
- Have the seller charge the pack to full before the appointment, and photograph the displayed range against the original EPA rating. That gives one independent read on degradation before a tool comes out.
- Pull State of Health with an OEM-level scan tool or a proven aftermarket EV tool. SoH is the number the whole verdict hangs on — the most expensive component's remaining life.
- Check pack warranty applicability against the VIN, mileage, and in-service date. A degraded pack inside warranty and one outside it are two completely different purchases.
- On the test drive, listen for drive unit whine and note any hesitation. Drive units have documented failure modes, and prior replacements should show in the service history.
- Test or date the 12V battery. It is cheap, it is mortal, and when it dies it strands the car no matter how charged the main pack is.
- Review the service records against the platform's known issues — on cars this well documented, prior repairs for MCU, drive units, and pack work are usually traceable, and their absence is information too.
- Accepting a charging demonstration, a voltage reading, or the dashboard range estimate as battery health. SoH from a real scan is the compression test of the EV world — without it you have inspected the most expensive component by vibes.
- Ignoring the 12V battery because the car is "electric anyway." A dead 12V strands the vehicle regardless of main pack charge, and on a used car of unknown history it is a cheap, near-term line item the buyer should expect.
- Skipping the pack-warranty lookup on a degraded battery. Whether coverage still applies to that VIN changes the finding from a five-figure risk to a manageable one — the SoH number without the warranty context is half a finding.
- Treating a used Tesla like a generic EV. Pre-2018 S/X MCU eMMC failure is a known $2,000-3,000 repair and drive units have their own documented modes — the known-issues list, checked against service records, is the inspection.
Two identical five-year-old Bolts: one scans 92% SoH, the other 78%. Why does the difference matter beyond today's range?
SoH drives both the range the buyer will live with and the resale value — the market prices degradation directly. The 78% car is also further along its degradation curve, closer to any warranty threshold, and its trend predicts a shorter useful life. Same model, same year, materially different purchases.
What two known items do you check on a 2016 Model S beyond battery health, and what do they cost?
The MCU — its eMMC flash memory wears out from constant logging on pre-2018 S/X cars, a $2,000-3,000 repair — and the drive units, which have documented failure modes you screen for by listening for whine and checking service history for prior replacements. Both are traceable in records because the issues are so well known.
The seller shows you a full charge displaying 240 miles of range. Why is that not enough to call the pack healthy?
The dash range estimate is a projection influenced by recent driving, temperature, and software — it is not a measurement of pack capacity. A scanned SoH figure is the real number; the displayed-range-versus-EPA comparison is a useful second read, but only alongside the scan, never instead of it.
Two identical five-year-old Bolts: one scans 92% SoH, the other 78%. Why does the difference matter beyond today's range?
Missed one? The reasoning above comes straight from the PPI exam bank, so this is the standard you will be held to.