How to fix Error 21 Centrifuge 5810R

Error 21 on a 5810R is tachometer not seeing RPM — the rotor is spinning, but the controller thinks it’s at “0” (your ER6 TK value confirms that).

That narrows it to three things:

Dirty/blocked tach sensor. The optical sensor under the motor gets dust and grease on it. Pull the rotor + bucket plate, clean the tach window and sensor with alcohol.

Tach pickup ring loose or damaged. The reflective ring/magnet ring on the motor shaft can slip. Make sure it’s still tight and intact.

Bad tach sensor or wiring. If cleaning doesn’t bring TK off “0,” the sensor itself is toast or the cable’s broken. Reseat both ends; replace the sensor if still dead.

If TK never moves above 0 after cleaning and reseating, you’re looking at tach sensor replacement. That fixes Error 21 almost every time.