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SV08: the screen shows «Error 8» (or 21 or 3): how to find the cause from the Mainsail message

Guide based on the manufacturer's official documentation; usi3d has not yet verified it on a machine.

Technology: FDM · Brand: Sovol · Symptoms: The printer shows an error code or message, Temperature error (MINTEMP, MAXTEMP, thermal runaway), A fan does not spin or is noisy · Updated on 9 October 2026

Symptom

The SV08 shows an error number on the knob screen (Error 8, sometimes 21 or 3) at power on or during a print. The number only says something failed: the actual cause appears in Mainsail, Klipper's web interface, with messages such as «Lost communication with MCU 'extra_mcu'», «ADC out of range», «Heater extruder not heating at expected rate», «TMC stepper_z», «Exception in Fan» or «Heater heater_bed not heating at expected rate».

Likely causes, from most to least likely

  1. extra_mcu: poor connection between the two printhead adapter boards or of the printhead communication cable on the mainboard; if it happens suddenly mid-print, the cable may have picked up interference.
  2. ADC out of range: abnormal nozzle thermistor reading (negative or far above normal), due to the thermistor or the adapter board.
  3. Heater extruder not heating at expected rate: damaged heater or thermistor wire, thermistor out of its clip, or an adapter board not supplying 24 V.
  4. TMC stepper (Z, Z1, Z2, Z3, X or Y): a motor or cable with a poor connection, interference or loose ground wires.
  5. Exception in Fan: the 2515 side fan on the heat sink (Fan1 port) or the adapter board; or the A/B belt end left too long and rubbing the fan.
  6. Heater heater_bed: the bed cable or module; or, on 220-230 V mains, a cold room when printing high-temperature materials; on 110 V mains the bed has less power.

Checks to tell which one it is

  • Connect the SV08 to the network, press «Show IP» on the knob screen and type that IP into the computer's browser to open Mainsail.
  • Copy the full Mainsail message and export the log from the time of the failure.
  • If Error 8 appears right at power on, carefully remove the nozzle cover and look at the adapter board light: if it turns red, tell Sovol.
  • For heating errors, set the nozzle to 200 °C or the bed to 65 °C from the screen and watch whether they heat.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

  1. extra_mcu: check the printhead connections

    With the printer switched off and unplugged, check that the two printhead adapter boards fit together with no bent, loose or damaged pins, and that the printhead communication cable is firmly connected on the base board (under the bottom cover).

    Check: At power on, Klipper starts without the extra_mcu warning.

    If that does not fix it: If «Can not update MCU 'extra_mcu' config» appears, restart Klipper or power cycle: according to Sovol it can happen when saving the configuration and is harmless. If it drops out during prints, ask Sovol for the cable or adapter board.

  2. ADC out of range or the extruder does not heat

    Restart, open Mainsail and check the nozzle thermistor reading: if it is negative or far above normal, tell Sovol (depending on the case they replace the thermistor or the adapter board). For «Heater extruder not heating at expected rate», with the printer cold check that the heater and thermistor wires are not damaged and the thermistor is still in its clip; if you replaced them, apply thermal paste.

    Check: The nozzle heats to 200 °C normally.

    If that does not fix it: Send Sovol the temperature reading and the log.

  3. Exception in Fan

    Heat the nozzle to 60 °C and check that the 2515 heat sink fan spins. With the printer off, check the cable in the Fan1 port of the adapter board; Sovol suggests swapping the Fan1 and Fan2 cables to tell whether the fan or the board is faulty. Also check that the A/B belt end is not so long that it rubs the fan.

    Check: The fan spins when heating and the error does not return.

    If that does not fix it: Replace the 2515 fan or the adapter board, according to the swap test.

  4. TMC stepper

    Check the motor and cable named in the message (according to Sovol, motor Z1 maps to the TMC Z error, Z2 to Z1, Z3 to Z2 and Z4 to Z3): firm connections and no damaged pins. Check that the yellow ground wires are tight and disconnect the camera, filament sensor and other accessories to rule out static electricity.

    Check: The printer moves the axes without the error.

    If that does not fix it: Sovol also describes changes in printer.cfg (Z motor current and silent mode); only make them if you know how to edit the configuration, or send the result to Sovol.

  5. Bed not heating or heating slowly

    With the printer off, check that the bed cable, its control module and the power cable are not loose or damaged. Then heat only the bed to 65 °C. If it heats slowly in a cold room when printing high-temperature materials, use an enclosure or a thermal cover.

    Check: The bed reaches 65 °C without error.

    If that does not fix it: If it does not heat at all, send photos of the wiring to Sovol.

  6. Error 21, Error 3 or if it persists

    Export the log from Mainsail and send it to Sovol with the message and the G-code you were printing. For Error 21, Sovol says to check the bed cables inside the base and, if it persists, to reflash the SV08 firmware.

    Check: Sovol can tell you the part or procedure.

    If that does not fix it: Follow their instructions.

Prevention

  • Note down your SV08's IP so you can read the full message in Mainsail when an error number appears.
  • Leave the A/B belt ends short enough not to reach the fan.
  • Do not plug or unplug anything with the printer switched on.

Still not fixed?

Tell usi3d's AI about your case: choose your printer, describe what you see and add photos if you like. It proposes checks step by step, with their sources.

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Printers it applies to

Sovol SV08.

Official sources

This guide summarises the official documentation linked here:

This guide is general information: usi3d has summarised it from the official documentation linked here, and it may not match your model, its version or its firmware. If in doubt, follow the manual and the support of your printer's manufacturer. You decide what you do with your machine, and working on it may affect its warranty.

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