SV08: the Z probe fails or the bed mesh is very uneven («probe triggered prior to movement», «No trigger on z…»)

Symptom

On the SV08 the print head hits the bed or does not stop when going down; QGL (gantry leveling) or homing stops with messages such as «probe triggered prior to movement», «No trigger on z after full movement» or «Endstop z still triggered after retract»; or the bed mesh in Mainsail shows very uneven heights.

Likely causes, from most to least likely

1. The gantry is out of square or slightly deformed and, during QGL, one corner is out of range.
2. The PEI plate is badly placed, resting on a positioning stop.
3. The proximity sensor cable is loose, damaged or short-circuited.
4. The proximity sensor is badly mounted (it should sit about 2 mm above the nozzle tip) or the nozzle is loose.
5. The proximity sensor is faulty and does not trigger, or stays triggered all the time.
6. The bed is curved or its fixing screws need adjusting.

Checks to tell which one it is

• With the printer on and the print head cover removed, look at the sensor light: at rest it is red and goes out when you bring a metal object close, such as a scraper or an Allen key.
• In Mainsail, go to MACHINE, click Refresh Endstops and check that the Endstop Z state changes when you bring metal close to the sensor.
• Check the bed mesh in Mainsail: according to Sovol, a range of up to 0.6 mm does not affect printing because the SV08 compensates for it.
• If the range is between 0.8 and 1.2 mm, measure the curvature of the cold bed with a ruler and strips of paper: if more than four sheets fit under the ruler, the deformation affects the first layer.

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. Check the PEI plate

   Check that the PEI plate is seated properly and is not resting on any positioning stop.

   Check: The bed mesh is more even when you level again.

   If that does not fix it: Check the sensor (next step).

2. Check the proximity sensor cable and mounting

   Switch the printer off, remove the print head cover and check that the sensor cable is not loose or damaged; unplug it and plug it back in. Check that the sensor is mounted correctly, about 2 mm above the nozzle tip, and that the nozzle is not loose.

   Check: The sensor light is red at rest and goes out when metal comes close.

   If that does not fix it: Test whether the sensor responds (next step).

3. Test the sensor

   Press Auto home and bring the steel PEI plate or another metal object close to the sensor. If the print head does not stop in Z, switch the printer off immediately to avoid damage. Also check the Endstop Z state in Mainsail as you bring metal close.

   Check: The print head stops and Endstop Z changes state.

   If that does not fix it: If the sensor does not trigger or stays triggered, record a video and ask Sovol for a replacement sensor.

4. Square the gantry and repeat QGL

   Push both ends of the X axis at the same time, never just one, all the way to the back of the gantry, so that the X/Y joints touch the A/B motor mounts with no gap. Then repeat QGL once or twice. If QGL stops with «probe triggered prior to movement», move the gantry up or down about 1 inch (2.5 cm) when the sensor is at the error position and recalibrate with QGL.

   Check: QGL completes and the bed mesh is no longer high on one side and low on the other.

   If that does not fix it: Adjust the bed screws (next step).

5. Adjust the bed screws

   If the mesh is still uneven, adjust the bed plate fixing screws with an Allen key: half a turn clockwise where the mesh shows red and half a turn anticlockwise where it shows blue. Check the mesh again after each adjustment.

   Check: The mesh range stays within what the SV08 compensates for (up to 0.6 mm, according to Sovol).

   If that does not fix it: If the range exceeds 0.8 mm, measure the bed curvature and send Sovol the mesh and the measurement.

6. Save the Z offset properly

   If the nozzle rubs or the layer does not stick after squaring the gantry, check the Z offset: an adjustment made during a print only applies to that print and is lost when you switch off. Save it after finishing or cancelling the print from the HDMI screen or from Mainsail, because the knob screen does not have that option.

   Check: The Z offset is kept after switching off and on.

   If that does not fix it: Contact Sovol with photos of the first layer.

Prevention

• Square the gantry by pushing both ends of the X axis at the same time, never just one.
• Save the Z offset from Mainsail or the HDMI screen after adjusting it.
• Seat the PEI plate properly, not on top of the stops.

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Printers it applies to

Sovol SV08.

Official sources

This guide summarises the official documentation linked here:

• The Z-axis probe is abnormal · Sovol
• probe triggered prior to movement · Sovol
• The hotbed is not level or the heightmap values vary greatly · Sovol