Antminer S21 Repair Guide: Common Issues and Solutions

The Antminer S21 represents Bitmain’s latest generation of SHA-256 mining hardware, delivering up to 200 TH/s at a remarkable 17.5 J/TH efficiency. Built around the BM1688 ASIC chip, the S21 is a significant leap forward in power efficiency and hashrate density. But with that increased complexity comes new failure modes that even experienced miners may not immediately recognize.

At D-Central Technologies, we have been repairing ASIC miners since 2016 — long before most repair shops existed. We have seen every generation of Antminer cross our bench, and the S21 is no exception. This guide covers the most common S21 issues we encounter, how to diagnose them, and when you should attempt a fix yourself versus sending your unit to our professional repair service.

Antminer S21 Specifications

Understanding the S21 Architecture

Before diving into failure modes, it helps to understand what makes the S21 different from its predecessors. The S21 uses four hashboards instead of the traditional three found in the S19 series. Each hashboard carries approximately 110 BM1688 chips arranged in voltage domains. The BM1688 is manufactured on a 5nm process node, which means tighter tolerances and more sensitivity to power delivery quality compared to the older BM1397 (S19) or BM1362 (S19 XP) chips.

The S21 also introduces a redesigned cooling architecture. The four-fan configuration (two intake, two exhaust) provides higher static pressure, which is necessary given the increased thermal density of the four-hashboard design. The control board runs an updated firmware with more sophisticated monitoring capabilities, including per-chip temperature sensing and real-time voltage domain monitoring.

This tighter integration means that problems in one subsystem — power, cooling, or control — cascade more quickly into broader failures. A marginal power supply that might have been tolerated by an S19 can cause immediate chip dropout on an S21.

Common Failure Mode #1: Hashboard Chip Failures

Symptoms

• One or more hashboards reporting reduced ASIC count (e.g., “found 98/110 chips” in the miner status)
• Kernel log shows chain[X] find Y chips where Y is less than the expected count
• Hashrate on the affected board is proportionally reduced
• In severe cases, the entire hashboard fails to initialize: chain[X] find 0 chips

Root Causes

BM1688 chip failures on the S21 can be caused by:

• Electrostatic discharge (ESD): The 5nm process makes BM1688 chips more susceptible to static damage during handling
• Thermal cycling stress: Repeated power on/off cycles in environments with large temperature swings can crack solder joints under BGA packages
• Voltage domain failure: A single dead chip can take out an entire voltage domain, causing multiple chips to appear “missing”
• Manufacturing defects: Early production runs of any new chip generation tend to have higher infant mortality rates
• Power surge or unstable input voltage: The BM1688 requires very clean, stable power. Voltage spikes can destroy chips instantly

Kernel Log Patterns

When diagnosing chip failures, the kernel log is your primary diagnostic tool. Look for these patterns:

# Normal initialization
chain[0] find 110 chips
chain[1] find 110 chips
chain[2] find 110 chips
chain[3] find 110 chips

# Partial chip failure (voltage domain issue)
chain[2] find 88 chips  <-- Missing ~22 chips = 1 voltage domain

# Complete board failure
chain[3] find 0 chips
ERROR: chain 3 failed to initialize

Repair Complexity: High

Chip replacement on BM1688 requires professional BGA rework equipment, a preheater, and replacement chips. This is not a DIY repair. The 5nm BGA packages require precise temperature profiles during reflow, and one degree too many can damage adjacent chips. If you are seeing chip failures on your S21 hashboard, send it to D-Central for professional diagnosis and repair.

Common Failure Mode #2: Fan Errors and Cooling Failures

Symptoms

• Miner status shows “Fan Error” or specific fan speed reading of 0 RPM
• Kernel log: ERROR: fan lost speed, fan_id=X
• Miner shuts down immediately after boot with temperature protection
• Audible change in fan noise — grinding, clicking, or complete silence from one fan
• Miner cycles between starting and thermal shutdown

Root Causes

• Fan motor bearing failure: The most common fan issue. The 14038 fans run at high RPM under load, and bearings degrade over time — typically after 15,000-25,000 hours of continuous operation
• Dust accumulation: Restricted airflow forces fans to work harder, accelerating bearing wear and reducing cooling efficiency
• Connector corrosion or loose connection: Vibration over time can loosen the fan connector on the control board, causing intermittent speed reading failures
• Fan controller IC failure on control board: Less common, but a failed fan controller chip will report all fans as failed even when they are physically spinning
• Cable damage: Crushed or pinched fan cables, especially if the miner has been shipped or moved

Kernel Log Patterns

# Fan speed read failure
fan_ctrl: fan[2] speed=0, expect >= 1000
ERROR: fan lost speed, fan_id=2

# Thermal protection triggered by inadequate cooling
WARN: chip temp 95C exceeds threshold 90C
ERROR: thermal protection triggered, shutting down chains

Repair Complexity: Low to Medium

Fan replacement is one of the most accessible S21 repairs. Replacement 14038 fans are relatively inexpensive and straightforward to swap. However, if the problem is a fan controller IC on the control board, that requires board-level soldering. Start by swapping the suspect fan to a different port — if the error follows the fan, replace it. If the error stays on the same port, the control board fan header or controller IC is the problem.

Common Failure Mode #3: Power Delivery Issues

Symptoms

• Miner fails to power on entirely
• One or more hashboards fail to initialize while others work fine
• Kernel log shows voltage out-of-range warnings
• Hashrate is unstable — fluctuates significantly over short periods
• Miner powers on but shuts down under load within minutes
• Visible damage to power connectors (melting, discoloration, carbon deposits)

Root Causes

• Inadequate power supply: The S21 draws up to 3,500W. Running it on an underpowered PSU or one with degraded output causes voltage droop under load
• Damaged power connectors: High current through the 6-pin connectors can cause overheating if connectors are not fully seated. Over time, this creates resistance, more heat, and eventual melting
• Hashboard voltage regulator failure: Each hashboard has step-down voltage regulators that convert the 12V input to the ~0.3V needed by the BM1688 chips. These regulators can fail from thermal stress or manufacturing defects
• Input power quality: Unstable grid power, frequent outages, or lack of surge protection can damage the power delivery chain
• Blown fuses on hashboard: The S21 hashboards have protective fuses that can blow during power surges, cutting power to sections of the board

Diagnostic Approach

Power delivery issues require methodical diagnosis with a multimeter:

1. Check PSU output voltage: Should be 12.0V – 12.6V under load. Below 11.8V indicates PSU degradation.
2. Inspect all power connectors: Look for signs of melting, discoloration, or carbon deposits. Check that all connectors are fully seated.
3. Measure voltage at each hashboard input: With the miner powered on, measure voltage at each hashboard’s power input. Significant difference between boards indicates a cable or connector problem.
4. Check for blown fuses: Using continuity mode on your multimeter, check fuses on the hashboard. An open fuse indicates a downstream short or surge damage.

Repair Complexity: Medium to High

Connector replacement and cable swaps are medium-difficulty repairs that a skilled home miner can handle with basic soldering skills. However, voltage regulator replacement and fuse diagnosis on the hashboard require professional equipment and knowledge of the S21 power distribution layout. If you suspect power delivery problems beyond simple connector issues, professional diagnosis is recommended.

Common Failure Mode #4: Temperature Sensor Faults

Symptoms

• Miner reports impossibly high or low temperatures (e.g., -40C or 150C)
• Temperature readings fluctuate wildly between refreshes
• Miner enters thermal protection mode despite adequate cooling and normal ambient temperature
• Kernel log shows sensor read errors or temperature values outside the valid range
• One hashboard consistently shows different temperature characteristics from the others

Root Causes

• Damaged temperature sensor on hashboard: Physical damage or ESD can kill individual temperature sensors, which are tiny surface-mount components
• Thermal paste degradation: The interface between the chip and heatsink degrades over time, leading to hot spots that trigger protection even when average temps are acceptable
• Sensor communication failure: The temperature data travels through a serial bus from the hashboard to the control board. A broken trace or bad connection can corrupt readings
• Firmware bug: Some early S21 firmware versions had incorrect temperature calibration tables, causing false readings

Kernel Log Patterns

# Sensor read error
chain[1] temp sensor read failed, using last known value
WARN: chain[1] chip[45] temp=127C (sensor error suspected)

# False thermal protection
chain[2] chip temp peak: 95C (threshold: 90C)
Thermal protection: shutting down chain[2]

Repair Complexity: Medium

If the issue is thermal paste degradation, re-applying thermal compound is a DIY-accessible repair, though it requires careful disassembly of the heatsink assembly. Sensor replacement on the hashboard requires micro-soldering skills and is best left to professionals. Always update to the latest firmware first — many temperature-related issues on the S21 have been resolved through firmware updates.

For a deeper understanding of how the control board and hashboards interact, see our guide on control board vs hashboard components.

Common Failure Mode #5: Control Board Issues

Symptoms

• Miner does not appear on the network (no IP assignment)
• Web interface is inaccessible or extremely slow
• Miner boots but does not start mining — all hashboards show “not ready”
• Ethernet link light is off or flickers
• Miner continuously reboots (boot loop)
• SD card recovery does not resolve the issue

Root Causes

• Corrupted NAND flash: The S21 control board stores firmware on NAND flash, which can become corrupted from power loss during firmware updates or from flash wear
• Failed Ethernet PHY chip: The physical layer chip that handles the network connection can fail, usually from ESD through the Ethernet cable
• RAM failure: Corrupted or failed RAM causes erratic behavior, boot loops, and mining failures
• Damaged SD card slot: If the SD card slot is damaged, firmware recovery becomes impossible through the normal method

Repair Complexity: Medium to High

Many control board issues can be resolved through SD card firmware flashing — a straightforward process that any miner can do. If SD card recovery fails, the problem is likely hardware-level (bad NAND, failed Ethernet PHY, or RAM), which requires board-level repair or control board replacement.

Common Failure Mode #6: Hashboard Communication Errors

Symptoms

• Hashboard is detected but reports 0 hashrate
• Intermittent hashboard dropout — board appears and disappears from miner status
• Kernel log shows communication timeout errors between control board and hashboard
• Error messages about SPI or I2C communication failures

Root Causes

• Damaged ribbon cable / data connector: The flat ribbon cable connecting the hashboard to the control board can be damaged during installation or shipping
• Oxidized connectors: In humid environments, the data connectors between hashboard and control board can oxidize, creating intermittent contact
• Cracked solder joint on data connector: Vibration and thermal cycling can crack the solder joints where the data connector is mounted to the hashboard PCB
• Failed signal buffer chip: The first chip in the communication chain on the hashboard can fail, preventing all downstream chips from responding

Diagnostic Approach

1. Reseat the ribbon cable: Power off, disconnect, clean the connector with isopropyl alcohol, and reconnect firmly
2. Swap hashboard positions: Move the suspect hashboard to a different slot. If the error follows the board, the problem is on the hashboard. If it stays on the same slot, the control board connector or port is at fault
3. Visual inspection: Use a magnifying glass or loupe to inspect the data connector solder joints on both the hashboard and control board

Repair Complexity: Low to Medium

Connector cleaning and reseating is easy and resolves a surprising number of communication issues. If the problem is a cracked solder joint, reflowing the connector requires moderate soldering skills. A failed signal buffer chip requires professional BGA rework.

Diagnostic Workflow: Systematic S21 Troubleshooting

When your S21 is not performing correctly, follow this systematic approach:

1. Read the kernel log. The kernel log is the single most valuable diagnostic tool. Access it through the web interface under System > Kernel Log, or via SSH. Our complete kernel log reading guide walks you through every message type.
2. Check the miner status page. Note which hashboards are online, their chip counts, temperatures, and fan speeds. Document everything before making changes.
3. Inspect physically. Power off the miner and visually inspect all connectors, cables, fans, and the PCBs for any obvious damage — burns, melted plastic, swollen capacitors, or corrosion.
4. Test with known-good components. If you have spare fans, cables, or a second S21, swap components to isolate the failure. This is the fastest way to narrow down the problem.
5. Check your power supply. Use a multimeter to verify PSU output voltage under load. Many S21 issues trace back to marginal power delivery.
6. Update firmware. Before going further, ensure you are running the latest firmware. Bitmain regularly releases updates that fix bugs and improve stability.
7. Try SD card recovery. If the miner is in a boot loop or not responding on the network, an SD card firmware flash often resolves control board software issues.

When to DIY vs. When to Send It to D-Central

Preventive Maintenance for the S21

Prevention is always cheaper than repair. Here are the maintenance steps we recommend for S21 operators:

• Clean dust every 3-6 months: Use compressed air to blow out accumulated dust from heatsinks and fan assemblies. A clogged S21 will thermal throttle and eventually cause chip damage.
• Inspect power connectors quarterly: Look for any signs of discoloration, looseness, or heat damage on all power connections.
• Monitor chip temperatures daily: Set up monitoring to alert you if any chip exceeds 85C. Catching thermal issues early prevents permanent damage.
• Use a quality surge protector or UPS: The S21’s BM1688 chips are sensitive to power quality. A surge protector is cheap insurance against catastrophic failure.
• Keep firmware updated: Bitmain releases updates that improve stability, fix bugs, and sometimes improve efficiency.
• Maintain proper airflow: Ensure intake and exhaust paths are clear. The S21 needs unrestricted airflow to maintain optimal operating temperatures.
• Document baseline performance: Record your normal hashrate, temperatures, and fan speeds when the miner is healthy. This makes it much easier to spot deviations early.

S21 Repair at D-Central Technologies

D-Central has been repairing ASIC miners since 2016. Our repair facility in Laval, Quebec is equipped with the specialized tools required for S21 board-level repair, including BGA rework stations, thermal imaging cameras, and diagnostic test benches. We stock BM1688 replacement chips and common S21 components to minimize turnaround time.

Our repair process includes:

• Full diagnostic report with kernel log analysis
• Component-level fault isolation
• Transparent quote before any work begins
• 48-hour burn-in test after repair to verify stability
• Detailed repair report documenting what was found and fixed

Whether your S21 has a single dead chip domain, a failed fan controller, or a complex power delivery issue, we have the expertise and equipment to bring it back to full hashrate.

Request an S21 repair →

Frequently Asked Questions

Related Resources

• D-Central ASIC Repair Services — Full repair service for all Antminer models
• Antminer Error Code Guide — Complete reference for every Antminer error message
• Kernel Log Reading Guide — How to interpret ASIC miner diagnostic logs
• Multimeter Guide for ASIC Repair — Essential electrical testing techniques
• Whatsminer Error Code Reference — Diagnostic reference for Whatsminer models
• Antminer S19 vs S21 Comparison — Head-to-head comparison of both generations