Pre-Scan vs Post-Scan: Compare the Evidence Before Closing the Repair

Two scan reports are useful only if the next reader can tell what was compared. A pre-repair report records the starting diagnostic state. A post-repair report records a later state. The shop still needs to explain the work between them, any missing coverage and the verification that supports closing the repair.

This guide focuses on collision-related scan comparisons and the handoff between technicians. The original worksheet is a documentation aid, not a vehicle-specific repair or release procedure.

Start with the manufacturer's requirements

GM's published collision position calls for a diagnostic scan during repair assessment and another after repairs, to check the addressed faults and whether repair work introduced new ones. It also identifies calibration or learn requirements for many replaced electronic components. Those operations follow the applicable repair information. See the GM pre- and post-collision scanning position statement.

That is a manufacturer-specific source. Check the current instructions and required diagnostic equipment for the actual vehicle and repair operations. Do not turn one manufacturer's position into a universal procedure for every make, or assume that any device producing a report provides the required coverage.

Preserve the starting report as its own evidence

Keep the original pre-scan export with its timestamp, vehicle identification, tool identification and reported results. Put technician interpretation in a separate note. A code observed before work is a starting finding; it does not by itself establish when the underlying fault began or whether the collision caused it.

Record any limits at the time of the scan. If a system could not be assessed, say which system and why, when known. Where damage prevents a safe assessment, follow the applicable safety instructions and document the gap. An unavailable result is not a clean result.

If the first saved report was produced after work had already begun, label its actual timing. Do not relabel it as a pre-repair baseline just to fill the worksheet. That missing baseline affects what can be concluded from the comparison.

Compare systems and statuses, not just code counts

An original comparison record should give each assessed system its own entry. For each entry, retain the module identification, exact DTC, displayed category or status, and a reference to the underlying report. Preserve unfamiliar labels as displayed instead of translating them into a meaning you have not verified.

Then classify the comparison in plain language:

• Reported in both records: identify the matching result and the unresolved interpretation or required next check.
• Reported only afterward: investigate the new observation using the applicable diagnostic procedure and the repair timeline.
• Reported only before: record that change, then identify what additional evidence establishes repair verification.
• Not comparable: explain the missing response, different scan scope or unavailable starting record.

These are worksheet categories, not DTC definitions. A smaller total code count is not enough to choose among them. Likewise, a new code after repair does not alone prove that the technician caused a new defect.

A fictional comparison with a missing system

Suppose the starting report contains results for systems A and B. The later report lists no codes for A but has no result for B. The repair folder also contains a completed work order without a separate record of a required calibration.

The tempting summary is that the post-scan is clear. The supported summary is narrower: A has no codes reported in the later record; B has not been compared; calibration completion evidence has not been located. No real vehicle, code or test specification is represented by this example.

The next action is to resolve B's scan coverage and establish the status of the required calibration, following the actual service information. This is more useful to the next technician than a green overall label.

Keep scanning and calibration completion separate

A saved code report records a diagnostic observation. A calibration, learn, inspection or functional check has its own required procedure and completion evidence. List the operations required by the repair information and attach their results separately. Do not use the absence of reported DTCs as the only record that those tasks were completed.

Before closing the case, reconcile the scan comparison with those task records and the original concern. Where an item remains unresolved, give it an owner and next action rather than describing the whole repair as verified. This guide does not authorize vehicle release.

Build a comparison another technician can use

• Vehicle and case reference:
• Applicable manufacturer instruction and revision:
• Pre-scan report reference, time and repair stage:
• Post-scan report reference, time and repair stage:
• Tool identification and scope for each scan:
• Per-system DTC, displayed status and comparison category:
• Coverage gaps and their explanation:
• Related repair operation and evidence:
• Required calibration, learn or check and completion record:
• Remaining item, responsible person and next action:

Repeat the per-system block in the download for each system being compared. Keep the original exports attached so a future reader can check the summary. The diagnostic report template can carry the customer-facing explanation; the scan tool and diagnostic software guide helps define which capability a shop is evaluating.

To discuss your shop's diagnostic records and handoffs, request STEP early access.