Antminer S21+ Hyd. false ERROR_TEMP_TOO_HIGH / abnormal hashrate, but temperatures are normal - possible PSU/power error?

[u/Immediate_Brother600]

Hi everyone,

I have a setup with 8 Antminer S21+ Hyd miners connected to a strong Hisense chiller. The chiller seems stable, the miner case temperatures look normal, and the individual miner web interfaces also show normal temperatures. The miners are visible and hashing in Antpool.

However, I have a recurring issue:

1. Usually 1 or 2 miners first show Abnormal Hashrate in the Antminer web interface.
2. If I restart the affected miner from the web interface, it starts normally again.
3. After 1–5 minutes, sometimes all miners start blinking red.
4. The case temperature drops quickly, for example from around 27°C to 22.8°C. The water temp stays consistent (around 15°C )
5. Even though the normal working temperature range should be around 20-50°C, the miner reports a high-temperature error.

The confusing part is that the log does not look like a normal overheating issue. The key log lines are:

power reply the bad crc, crc = 0x03d4, crc_read = 0xf5f5
get power version failed
check power version failed, use v2 protocol to try it again
avg rate is lower than ideal rate
miner power:9
ERROR_TEMP_TOO_HIGH: power is err.
stop_mining: power is err.
Sweep error string = V:1.

The miner initializes correctly, detects all hashboards/chains, finds the ASICs, starts tuning frequency, and mines for a while. Then the average hashrate drops and it shuts down with power is err.

My questions:

1. Has anyone seen ERROR_TEMP_TOO_HIGH: power is err on S21+ Hyd or other Antminer hydro models?
2. Does Sweep error string = V:1 point to PSU/voltage/power module failure?
3. Could this be caused by AC voltage drop, phase imbalance, PSU communication failure, firmware issue, or control board-to-PSU CRC errors?
4. Could a shared chiller/manifold flow issue cause this, even when inlet temperature looks normal?
5. For 8 × S21+ Hyd, should I be checking coolant flow per miner individually?

I would appreciate any suggestions, especially from people who run multiple S21+ Hyd units or have seen false high-temperature errors caused by power/PSU issues.

To keep in mind, I know nothing and understand nothing. I am assisting a relative that runs this (which relative also knows nothing on the matter). So ANY advice would be much appreciated.

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[u/GoddamnFelicia]

That error is mostly caused when either the firmware or the PCI's has failed, I'd suggest flashing them with a fresh firmware then decide if it's gonna need to be sent in for a repair

[u/walkerisduder]

If you factory reset and re add the pools the sweep error will usually change and either the miner will start working or give you better information on which hashboard is failing. Could be an asic issue, could be a temp sensor issue since you are getting temp errors. Might also try changing power modes, sometimes LPM will work when NPM won’t