Out-of-Tolerance Calibration: What to Do When Equipment Fails

An instrument has come back from calibration and the results show it was measuring outside its specified tolerance. Now what?

This is one of the most common — and most poorly handled — situations in calibration management. ISO 9001:2015 Clause 7.1.5(c) specifically requires that when equipment is found not conforming to requirements, you "determine if the validity of previous measurement results has been adversely affected" and "take appropriate action."

Here's the step-by-step procedure that auditors expect to see.

Step 1: Identify the Affected Period

The out-of-tolerance condition started at some point between the last known good calibration and the current failed calibration. You need to define this window:

• Start of affected period: The date of the last calibration where the instrument passed (was within tolerance)
• End of affected period: The date of the current calibration where it failed
• Duration: This is the period during which measurements made with this instrument may be unreliable

Document this window clearly. It defines the scope of your investigation.

Step 2: Assess the Magnitude of the Error

Not every out-of-tolerance result has the same impact. A micrometer reading 0.002mm outside tolerance is different from one reading 0.05mm outside tolerance.

Record:

• The measured deviation: How far outside tolerance was the instrument?
• The tolerance requirement: What tolerance was specified for the measurements this instrument was used for?
• The ratio: Was the deviation a small fraction of the tolerance, or a significant portion?

If the deviation is less than 10% of the tolerance band for the measurements being made, the impact on product conformity may be negligible. If the deviation exceeds 50% of the measurement tolerance, the impact could be significant.

Step 3: Trace Affected Measurements

Identify everything that was measured with this instrument during the affected period:

• Which products, batches, or test results were measured?
• Which production orders used measurements from this instrument?
• Were any of these measurements used for acceptance/rejection decisions?
• Were any measurements close to tolerance limits (where the instrument error could have changed the pass/fail outcome)?

This is where spreadsheet-based calibration tracking breaks down. Without a systematic link between instruments and measurements, this tracing exercise becomes guesswork.

Step 4: Evaluate the Impact

For each set of affected measurements, determine whether the out-of-tolerance condition could have caused incorrect results:

• No impact: The measurement tolerance is much wider than the instrument deviation. The instrument error would not have changed any pass/fail decision.
• Possible impact: The instrument deviation is significant relative to the measurement tolerance. Some results near the tolerance boundary may have been incorrectly accepted or rejected.
• Confirmed impact: Re-measurement of retained samples or products confirms that results were affected.

Document your evaluation for each affected measurement set. "No impact — instrument deviation of 0.003mm is within 5% of the 0.1mm measurement tolerance" is the level of detail auditors expect.

Step 5: Take Appropriate Action

Based on your impact assessment:

• No impact determined: Document the assessment and close. No further action required on the products, but consider adjusting the calibration interval (see Step 7).
• Possible impact: Re-measure retained samples or products if available. If re-measurement is not possible, assess the risk and make a disposition decision (accept, quarantine, reject, recall).
• Confirmed impact: Quarantine affected product. Re-measure. Reject or rework as appropriate. Notify the customer if affected product has already been shipped.

Step 6: Document Everything

Your out-of-tolerance investigation record should include:

• Instrument identification (ID, serial number, type)
• Calibration certificate number and date
• Nature and magnitude of the out-of-tolerance condition
• Affected period (dates)
• List of affected measurements, products, or test results
• Impact assessment for each
• Disposition decision (accept, quarantine, reject, re-measure)
• Corrective actions (recalibration, interval adjustment, instrument replacement)
• Sign-off by the responsible person

This record is what your auditor will ask for. A complete, well-documented investigation — even one that concludes "no impact" — demonstrates your quality system is working.

Step 7: Prevent Recurrence

After closing the investigation, consider:

• Shorten the calibration interval for this instrument type if the out-of-tolerance condition suggests the current interval is too long
• Review similar instruments — if a micrometer drifted, check whether other micrometers of the same type and age should be calibrated early
• Investigate the root cause — was the instrument damaged, stored improperly, used outside its range, or simply at the end of its useful life?
• Update your calibration schedule with the revised interval

Sources

• ISO 9001:2015 — Quality management systems (Clause 7.1.5)

This guide provides a general out-of-tolerance investigation procedure. Your specific procedure should be documented as part of your quality management system and approved by your quality manager. This is not legal or compliance advice.