Diagnostic reading guide

How to read a used EV diagnostic report

Translate SOH, cell voltage delta, insulation resistance, fault codes, charge counts and temperature distribution into useful buying questions.

SOH

Capacity-health trend. It is not one trip range and not the same as an official warranty conclusion.

Ask: Ask for equipment, read time, SOC, temperature and any historical comparison.
Caution: One SOH number cannot be interpreted without model year, trim and reading conditions.

Cell voltage delta

Difference between maximum and minimum cell voltage inside the pack, used as a consistency signal.

Ask: Ask for current SOC, rest/load state, max/min cell IDs and whether it repeats.
Caution: Delta can move at low SOC or after load; abnormal readings need fault-code review.

Insulation resistance

High-voltage insulation safety between HV system and body, especially important after flood, impact or repair.

Ask: Ask for test conditions, equipment report, HV fault codes and water-damage repair history.
Caution: Insulation abnormality is a safety risk and should not be closed on verbal explanation.

Historical/current fault codes

Historical codes show what happened before; current codes show whether the issue remains.

Ask: Ask for full screenshots, before/after clearing, post-test-drive recurrence and related modules.
Caution: “Codes cleared” is not enough; review original records and recurrence.

Fast/slow charge counts

Helps judge usage intensity and thermal stress when combined with mileage, city and age.

Ask: Ask for fast/slow charge counts, recent charging locations and hot-weather power limiting records.
Caution: Count alone is not a verdict; high fast-charge share plus heat and mileage is more concerning.

Battery temperature distribution

Shows whether thermal management is balanced and whether single-point abnormal heat exists.

Ask: Ask for temperatures before/after drive, max/min temperature gap, cooling repairs and fault codes.
Caution: Temperature gaps need context: ambient temperature, charging state and cooling-system condition.

Review order

Do not stare at SOH alone

Review in this order: reading conditions, key metrics, fault codes, test-drive recurrence and contract conditions. This prevents over-interpreting one number.

Reading conditions

SOC, temperature, rest time and pre/post drive state affect readings.

Full report

Screenshots are not enough; keep the full diagnostic report where possible.