How do you calculate diagnostic time for returned units?

Multiply units in the queue by the average diagnostic time per unit, then multiply by (1 + NFF allowance / 100). For 35 units at 90 minutes each with a 20% allowance the result works out using the base and the allowance shown.

Why include an intermittent fault and NFF allowance?

No-fault-found and intermittent returns take far longer than clean failures because the technician must reproduce a fault that may not appear on the bench. A 20% allowance pads the base time to account for this extra digging.

What is a no-fault-found (NFF) return?

A unit returned as faulty that passes all bench tests. NFF returns are common in field electronics and consume disproportionate diagnostic time, which is why they get a dedicated allowance here.

How can I cut returned-unit diagnostic time?

Capture better field failure data, use guided diagnostic scripts, automate first-pass screening, and feed recurring fault patterns back to design so the population of hard-to-find returns shrinks.

Does this include the actual repair time?

No. This figure covers diagnosis and fault-finding only. Add separate rework, part-replacement, and retest times to get full RMA turnaround.

What's a reasonable diagnostic time per unit?

It depends on complexity and documentation, from 15 minutes for a well-instrumented board to a few hours for a sealed multi-board instrument. 90 minutes is a fair average for a mid-complexity control unit.

Measurement, Test & Control Equipment calculator

Returned Unit Diagnostic Time Calculator

Returned unit diagnostic time is the total bench hours needed to triage and fault-find a queue of returned instruments or control units in a repair or RMA cell. Repair-depot supervisors and field-service managers use it to plan technician staffing, set realistic RMA turnaround commitments, and clear backlogs. It matters because intermittent faults and no-fault-found (NFF) returns are notoriously time-consuming: a unit that fails in the field but passes on the bench can soak up far more time than a hard failure, so a separate NFF allowance keeps estimates from being wildly optimistic. The metric turns a stack of returned boxes into a defensible hours number you can staff against.

What this calculator does

Estimate total labor hours needed to diagnose a batch of returned or failed units, including fault isolation, functional test, and root cause documentation. Plan repair depot capacity and technician workload.
Use when planning repair depot staffing, estimating turnaround time for RMA units, or budgeting diagnostic labor for warranty returns of test and measurement equipment.
It multiplies the number of units awaiting diagnosis by the average diagnostic time per unit, then adds an intermittent-fault and NFF allowance to give total diagnostic hours.

Formula used

Base diagnostic time = units awaiting diagnosis x average diagnostic time per unit
Total diagnostic time = base diagnostic time x (1 + intermittent fault allowance / 100)

Inputs explained

Units awaiting diagnosis:
Average diagnostic time per unit:
Intermittent fault and NFF allowance:

How to use the result

Use it when planning repair-cell staffing, committing to RMA turnaround times, or sizing the effort to clear a returns backlog.
It assumes one average diagnostic time across all units; a queue dominated by hard, repeatable failures will finish faster, while one heavy with intermittent faults can blow past even the NFF allowance.

Current U.S. benchmarks

Steel mill PPI stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. New factory orders are up 2.3% year over year (Census).

Common questions

How do you calculate diagnostic time for returned units? Multiply units in the queue by the average diagnostic time per unit, then multiply by (1 + NFF allowance / 100). For 35 units at 90 minutes each with a 20% allowance the result works out using the base and the allowance shown.
Why include an intermittent fault and NFF allowance? No-fault-found and intermittent returns take far longer than clean failures because the technician must reproduce a fault that may not appear on the bench. A 20% allowance pads the base time to account for this extra digging.
What is a no-fault-found (NFF) return? A unit returned as faulty that passes all bench tests. NFF returns are common in field electronics and consume disproportionate diagnostic time, which is why they get a dedicated allowance here.
How can I cut returned-unit diagnostic time? Capture better field failure data, use guided diagnostic scripts, automate first-pass screening, and feed recurring fault patterns back to design so the population of hard-to-find returns shrinks.
Does this include the actual repair time? No. This figure covers diagnosis and fault-finding only. Add separate rework, part-replacement, and retest times to get full RMA turnaround.

Last reviewed 2026-05-12.