Antminer T21 – Fan Speed Error

Hard power cycle the T21 at the breaker for 60 full seconds, then boot. Not a soft reboot — full power off. This clears any wedged fan-driver state in the control-board firmware. Roughly 15% of T21 fan errors in D-Central's repair queue clear on a hard cycle alone, because the tach-count software got into a bad state during a transient voltage dip.

Open the chassis with the miner off and check every fan is physically clear. Look for stuck debris: cat hair, dryer lint, cardboard, a loose cable sucked into a blade. Remove any obstruction with a lint-free brush and a shop-vac. Do NOT use canned air inside the chassis while fans are mounted — it spins blades beyond their rated RPM and fries bearings.

Reseat all four fan 4-pin JST connectors firmly. Unplug each, inspect the pins for oxidation or bending, plug back in until the locking tab clicks. This is the single most common T21 fan-error fix — intake vibration walks the connectors out over 30-60 days. Pay extra attention to the two exhaust-side connectors; they vibrate hardest.

Verify intake airflow is unobstructed. Nothing within 20 cm of the intake grille — no shelving, cloth, or wall. The T21 needs roughly 700 CFM of through-flow at nameplate 190 TH/s and cannot make that parked against a basement wall. Clear the intake plane and re-boot.

Check ambient temperature at the intake with an IR thermometer — at the grille, not room-middle. Target ≤ 30 °C, hard ceiling 35 °C. A T21 in a hot room will throttle fans to max immediately on boot; some firmware builds misinterpret sustained-max as a fault and false-trigger the fan-lost error.

Swap the suspect fan with a known-good T21/S21 fan. Power off at the breaker, remove the four chassis screws holding the fan shroud, swap fans, reassemble, boot. If the fault moves to the new fan location → control-board port problem. If it clears → the original fan was dead. Part is a 12038 12V 4-pin PWM dual-ball-bearing fan. Budget $25-45 CAD per fan.

Multimeter-check the 12 V rail at the control board while the miner is trying to boot. Expect 12.0 ± 0.3 V at idle and ≥ 11.8 V during fan spin-up. Sagging below 11 V points at the PSU or the PSU-to-control-board cable. Swap the PSU with a known-good APW12/APW17-class unit and recheck. Multiple fans faulting simultaneously almost always traces to rail sag, not to four independent fan failures.

Test each control-board fan port with one known-good fan — one port at a time. If a specific port refuses to detect a known-good fan, its driver MOSFET or pull-up has failed. Mark the dead port. You can keep operating on the remaining good ports at reduced hashrate or route that port to repair. Continuing to plug new fans into a dead port costs you fans.

Inspect the suspect fan's yellow tachometer wire. With the fan off, wiggle the yellow wire at the fan hub and at the connector. A broken-inside-the-sleeve tach wire moves freely with no resistance; a healthy wire feels bonded. If the fan moves air but the tach reads 0 RPM, the wire is broken — resolder a pigtail for under $5 CAD or replace the fan.

Clean the fans without disassembling the bearings: soft brush on the blades, IPA-dampened lint-free wipe on the hub and blade roots. Never submerge a fan; never spray anything into the bearing seal. Dust mass imbalance on the blades is a real cause of RPM drift — a thick dust crescent on one side of the impeller pulls measured RPM down 5-8% and can flag a false fan-lost event.

Flash DCENT_OS on the T21 — D-Central's own open-source Antminer firmware, built by Mining Hackers for Mining Hackers, fully open-source, maintained at github.com/DCentralTech/DCENT_OS. For fan diagnostics it exposes per-fan RPM history, a configurable RPM floor (useful if you run sub-nameplate), and PWM duty-cycle logging. Alternatives: Braiins OS+, LuxOS, Vnish — all three also report per-fan detail.

If Step 8 isolated a dead fan port, rework the fan-driver MOSFET on the control board. Typical package is SOT-23 or SO-8 depending on T21 control-board revision. Desolder with hot air at 310-330 °C, replace with a matched part, reflow. 30-minute bench job with a rework station and a steady hand. MOSFET itself: $1-3 CAD; bench time: yours.

Reflow any cold-solder joint at the 4-pin JST through-hole pads on the control board. Vibration-induced cold joints show as a hairline ring around the pin under a loupe. Apply flux, add fresh solder, reflow. This is why D-Central's bench protocol includes a loupe check of every fan connector before we declare a control board dead.

Re-terminate a damaged fan cable end. If the fan is good but the 4-pin JST connector is cracked, has a bent pin, or a pin has backed out, snip the connector, strip 5 mm, crimp new JST pins, assemble into a fresh housing. A kit of 4-pin JST crimp terminals + housings is about $15 CAD for a lifetime supply.

Firmware cross-check: compare your installed T21 firmware version against Bitmain's current release notes at support.bitmain.com/downloads. Known fan-threshold bugs are listed when fixed. Roll forward or back one version, observe for 30 minutes at steady-state load. If the fault was a firmware false-positive, this catches it. If not, you've at least ruled firmware out before pulling the chassis apart.

Stop DIY and ship to D-Central when: two or more fan ports show the same failure signature, the fan-driver MOSFETs on the control board have visible scorch or discoloration, 12 V rail sag persists through a known-good PSU swap, or you've replaced the same physical fan twice in 90 days. These are board-level issues — replacing more fans won't fix them. Book an ASIC Repair slot at d-central.tech/services/asic-repair/.

D-Central bench process: programmable-load control-board test fixture exercises all four fan channels independently; MOSFET-level repair of failed driver circuits with matched replacement parts; full 4-pin JST connector refresh when wear is detected; 24-hour burn-in at nameplate 190 TH/s with all four fans RPM-logged. Standard turnaround 5-10 business days. Canada / US / international shipping welcomed.

Ship the T21 (or control board only, if that's what we agreed to repair) safely: anti-static bag around any exposed PCB; original foam plus a double-box with at least 5 cm of padding every side for the full miner. Include a printed note listing observed RPM values, firmware version, and which fan/port faulted. This saves us diagnostic time and saves you repair cost — it is literally the cheapest step in the process.