Antminer S21 Pro / S21+ / S21 XP Missing Chips — "Only Find X ASIC": Causes & Fix

Reading the chip count

The S21 Pro, S21+ and S21 XP are the most widely deployed current-generation Antminers, and a low chip count is one of the most common faults they come in with. Every time the miner starts, the control board counts how many chips answer on each of the three hashboard chains and writes the result to the kernel log. The number tells you a lot:

Always compare the count with the correct full chip count for your exact model and board revision. S21-series variants differ, so a "missing" number is only meaningful against the right reference.

Common S21 Pro / S21+ / S21 XP log messages

People often search for these lines word for word. Exact wording can vary slightly between firmware versions.

Checks you can do yourself

1. Save the kernel log and write down the chip count on each chain. Power-cycling erases it.
2. Check the PSU and grounding. Bitmain's own guidance for missing chips is to try a different power supply and make sure the miner is properly grounded. Use the correct PSU model for your S21 variant.
3. Reseat the data and power connections on the affected board after powering off, and look for heat marks on the connectors.
4. Swap the board to another slot. If the low count follows the board, the fault is on the board.
5. Check temperatures. A count that drops only after the miner warms up points to heat-fatigued joints, not a configuration problem.

If the count stays low on the same board with a good PSU, good grounding and good cables, the board needs chip-level repair. Don't keep power-cycling it — a damaged domain can take healthy parts with it.

How we repair S21-series boards with missing chips

S21-series hashboards are dense, high-power boards, so we diagnose before we apply full power. On the test fixture we measure every voltage domain for shorts and opens, use the chip count to narrow the search to a domain or chip position, then confirm the failed part under load with thermal imaging. The faulty chip or power component is replaced, the domain is re-measured, and the board runs a full-load burn-in before it goes back. You can see a worked example — an S21 Pro board with a whole voltage domain gone dark — on our S21 Pro / S21+ / S21 XP repair page.

A solo miner is typically repaired in 1–3 days, and small batches within 1–2 weeks. Running the standard S21 or T21? See our S21 / T21 repair service. Power supply trouble on the same units? We also repair APW17 power supplies.

Frequently asked questions

What does "Chain X only find N asic" mean on an S21 Pro?

At start-up the control board counts the chips that answer on each hashboard chain. If the count is lower than the board's full chip count, the chain breaks part-way along — typically a failed chip or a dead voltage domain at that position. If the count is 0, nothing on the chain answered and the miner switches that board off.

Why does the missing-chip count matter?

The chips sit in a chain and in groups that share a voltage domain. A count that stops at a consistent position, or a whole group missing at once, points straight at the failed chip or the domain that lost power. It saves a lot of time on the bench.

Can a bad power supply cause missing chips on an S21?

Yes. Bitmain's own advice for missing chips includes trying a different power supply and making sure the miner is properly grounded. Use the correct PSU model for your S21 variant — the high-power S21 models need a supply that can deliver their full current.

Is it safe to keep running an S21 Pro with missing chips?

It will usually keep hashing at reduced speed, but a failing chip or a damaged voltage domain can get worse and take healthy parts with it. Save the log, do the basic checks, and get the board diagnosed rather than running it indefinitely.

How long does S21 Pro missing-chip repair take?

At Frontline Hash, a solo miner is typically repaired in 1–3 days and small batches within 1–2 weeks, depending on the fault and parts. On-site repair is also available for farms.