How to use an OBD scanner when buying a used car | DVI Reports

Scan before and after a test drive, understand cleared codes and readiness, and combine a VIN report with a pre-purchase checklist.

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How to use an OBD scanner when buying a used car

Illustrative image: Handheld OBD-II scanner and connection cable

Use diagnostics with the seller’s permission as part of an independent inspection. A basic scanner can provide useful clues, but cannot certify engine condition, mileage or airbag integrity.

The guide in 36 seconds

  • Zero OBD faults? The car may still have problems

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Video in Romanian. Opens in a new tab; the full steps and explanations are below.

Buying guide Editorial review: 17 October 2026

At a viewing, read and save. Do not clear faults, reset adaptations or run regeneration and actuator tests.

1. Prepare the viewing and scanner

  1. Ask for the VIN, engine and a cold start. Confirm scanner compatibility and module access for the exact car.
  2. Get permission to scan and save a report. Generic emissions OBD access does not guarantee ABS, airbag, transmission or traction-battery access.
  3. Follow the vehicle and adapter manuals when connecting. Work while parked in a safe, ventilated place.
Sources: OBDLink — enhanced diagnostic coverageADAC — used-car checklist (PDF)

2. Save the first scan

  1. Compare the VIN on the car and documents. If diagnostics also reports it, record discrepancies; an OBD reading does not certify identity.
  2. Save each code with its module, description and state: stored, pending or permanent where supported.
  3. Export the report and freeze-frame before intervention. Keep data locally and mask the VIN before publishing screenshots.
Sources: OBDLink — diagnostics / freeze-frame

3. Look for signs of a recent reset

Readiness monitors show whether emissions self-tests have completed. Several incomplete tests may follow code clearing or battery disconnection, but can also mean driving conditions have not been met. They do not alone prove deception.

  1. Distinguish not ready/incomplete from not supported. An unsupported test is not a failed one.
  2. Ask why a reset occurred and for repair documents. Distance or time since clearing can help only if supported.
  3. A permanent code can remain after repair until the ECU verifies the fix. It cannot simply be removed with the clear button.
Sources: BAR — OBD readiness & permanent DTC (principii tehnice; regulile de inspecție sunt pentru California)

4. Test drive and second scan

  1. Only drive if the vehicle’s condition permits; oil pressure, overheating, abnormal braking or severe rough running require stopping the check and seeking workshop help.
  2. The driver watches the road. A passenger or technician can log data; do not provoke faults with harsh acceleration.
  3. Rescan afterwards and compare reports. A short fault-free drive does not exclude intermittent problems or necessarily complete every monitor.
Sources: Ford — warning lightsBAR — OBD readiness & permanent DTC (principii tehnice; regulile de inspecție sunt pentru California)

5. Combine diagnostics, history and inspection

carVertical is one example of a complementary commercial report. Check pricing, coverage and terms before paying; we do not promise that it includes every accident. The source link is informational and contains no affiliate identifier.

On a phone, scroll the table to see all columns.

5. Combine diagnostics, history and inspection
CheckCan help withCannot establish alone
OBD scanReported faults and operating conditionsAbsence of all faults, true mileage or structural integrity
RAR / documents / carVerticalA chronology of available recordsAbsence of unrecorded events
Independent inspectionObserved and measured vehicle conditionA guarantee against future failures
Sources: carVertical — limitele raportului de istoricADAC — used-car checklist (PDF)

6. Decide using an inspection report and quote

  1. Request diagnosis and a written estimate for active or unexplained faults before negotiating.
  2. Defer the purchase for safety warnings, identity discrepancies or refusal of an inspection.
  3. For signs of recent clearing, request reassessment after manufacturer-prescribed operating conditions. No universal mileage figure is a guarantee.

Your checklist

Tick off completed checks. The list resets when you reload the page.

0 / 7 checks completed

I have permission to scan. I checked the VIN and documents. I saved codes, status and modules. I saved freeze-frame and readiness. I compared scans before and after the drive. I cross-checked the VIN report with records. I have an inspection report and estimate before paying.

Frequently asked questions

Does zero codes mean a good car?

No. Mechanical defects, inaccessible modules, cleared codes and faults absent during the check can remain.

Can an ELM327 certify mileage?

No. Available readings vary and a figure from one module does not certify true mileage. Compare dated records and seek specialist checks.

Should I clear codes to see what returns?

Preserve evidence at a viewing. Clearing can remove useful data; reassessment after repair should be documented and agreed with the owner.

Do you offer vehicle inspections, history reports or transport?

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