Is a same-size fan an equivalent replacement?

No. Equal frame size, voltage and bolt pattern make a fan a candidate, not an equivalent. Two 40×40×20 mm fans from different makers can differ by 30% or more in delivered flow at the same resistance, differ in tach pulse counts, PWM frequency windows, start-up behaviour and noise — and only a validation on the customer's unit can close the file. Never endorse 'same outline dimensions = equivalent'.

What 'same size' actually matches

Mechanical envelope: length × width × thickness, hole pattern, and sometimes airflow direction. That is where the equivalence ends. The P-Q curve shape depends on impeller geometry, motor design and inlet structure; two same-size fans intersect the same system curve at very different flows. A 20% difference in free-air CFM routinely becomes 30–40% difference in installed flow on a resistance-dominated path.

The interface traps that surface late

Tach output with a different pulses-per-revolution makes the host report the wrong RPM. A PWM input that expects 25 kHz but distorts at your controller's frequency can misbehave. An RD pin with inverted logic can turn a locked-rotor alarm into a false healthy signal. A connector with the same pitch but different latch geometry passes the bench and fails the production line. Each of these has caused field escapes — all are caught by a written qualification checklist.

• Compare tach PPR and pull-up scheme before building samples.
• Verify PWM duty-to-speed mapping at 20/50/100% duty.
• Dry-run the connector mating on a production fixture, not by hand.

The qualification that closes the file

Freeze a checklist: mechanical fit, airflow direction, electrical (voltage range, steady and start-up current), signals, acoustics, vibration, thermal validation on the unit at worst-case duty, and the customer's change-control requirement. Run first-article samples through it, document results, then release. MEGA Tech validates replacement candidates exactly this way — samples are proven on the customer's unit, not on a datasheet comparison.