Temperature sensor disconnected

One of the three thermistors on the barrel reports a temperature that cannot be real, and Klipper stops heating that zone. This is the page for finding the break. What the sensor is, where it sits and how it is mounted is on the component page: Temperature sensor NTC 100k.

The bed is a separate case

This page is about the three barrel zones. The bed has its own sensor circuit, and two sensors with only one of them plugged in, so a bed reading -99 °C or flickering starts at a different connector — see The bed has two temperature sensors.

Symptoms

• One zone reads an impossible value — deeply negative, often flickering, commonly around -90 °C — or jumps by tens of degrees for no thermal reason
• Klipper raises extruder below minimum temp, temperature not valid, or not heating as expected, and refuses to heat that zone
• The print stops, or the machine will not start heating at all
• Sometimes visible: a sensor wire hanging loose at the extruder, or the brass insert backed out of the barrel

It is not the sensor

Start from the right assumption

A failed thermistor has never been the cause. Every temperature fault on the G1 has come back to one of two things:

1. A connector in the signal path — the JST junctions were the historic weak point of the wiring
2. A sensor whose calibration is not set in firmware — each sensor carries its own three calibration points, and a swapped sensor without them reads wrong

Replacing the sensor without fixing one of those moves the same fault to a new part.

There is a third, mechanical case at first assembly: the brass insert backing out in shipping and taking the sensor with it. That is a loose part, not a failed one.

Why it reads far below zero

An NTC’s resistance falls as it gets hotter. Break the circuit anywhere between the Octopus board and the sensor and the board sees infinite resistance, which the curve maps to the bottom of the scale. The firmware minimum is 0 °C, so the reading is rejected and the zone is refused.

What you actually see is a negative reading, absurdly low, usually flickering. The figure often lands around -90 °C, and that is the number quoted through this wiki — but treat it as indicative, not as a value to match. An open circuit does not settle on one temperature. The diagnosis is the sign and the magnitude: negative, and nowhere near anything physical.

So do not read a steady value as one fault and a jumping value as another. Both are the same finding — the reading is not arriving — and the work is the same: find the break.

Finding the break

The signal runs sensor → JST at the extruder → ethernet cable from the extruder breakout to the panel box → JST on the Octopus. Check in this order, because that is the order they fail in:

1. The JST right after the sensor, at the extruder — the most common one
2. The JST on the Octopus, inside the panel box
3. The ethernet cable between the two — every pin fully seated at both ends

The wiggle test. Bring the zone toward temperature, watch its graph in Mainsail, and gently move each connector in turn. The reading must not move. If it jumps when you touch one connector, that connector is the fault.

Fixes

The insert has backed out

The sensor sits in a brass insert threaded into the barrel. Screw the insert back in — turn the insert only, never the wire — reconnect, and check the reading at room temperature before heating.

A connector is unplugged

Reseat it fully and check the reading before heating. If the same connector works loose more than once, stop reseating it: a JST that will not hold is the fault itself, and the fix is the sensor kit below.

A connector passes the wiggle test badly

Reseat it and re-check. If the jumping comes back, the crimp is the fault, and a bad crimp cannot be inspected without destroying the housing. Do not chase it.

Two ways out, one temporary and one permanent:

• Splice past the JST. Cutting the connector off and wiring the sensor straight through to the breakout works, and gets a machine printing again in the field. It works precisely because it removes the JST
• The sensor kit. Contact Ginger Additive support: on machines with the older JST sensors, support ships the current sensors directly. That is the resolution, not a repair on the bench

The wire is damaged

Check the kit first. A new machine ships three spare thermistors in the tools and screws box, under Spare Parts, with the spare Z probe switch. If none is left, request one from support.

A replacement sensor needs its calibration set

Each sensor carries three calibration points that live in that machine’s printer.cfg. A swapped sensor without its values reads wrong. Get the values with the sensor — see the current sensor.

The reading drops only when the fan runs

A different fault that produces a similar alarm. The nozzle zone drops 20–30 °C the moment the part-cooling fan ramps up, and Klipper shuts down for a heating error. The sensor is fine — it is being cooled by the fan, on machines where it sits outside the barrel insulation.

1. Rotate the fan toward horizontal, away from the sensor
2. Fit the sensor kit on an older machine. It moves the sensor inside the insulation on a metal cartridge, which is the root fix — see why the cartridge matters

Telling the two apart: a fan-induced drop tracks fan speed and recovers when the fan stops; a bad connection jumps at random, including to -90.

The sensor kit

The standing fix for recurring sensor trouble: three sensors in the metal cartridge that carries the tip inside the barrel insulation, with the calibration values to write into the machine’s printer.cfg. It removes the JST weak point and sets the calibration in one go. Request it from Ginger Additive support.

After the fix

1. Firmware Restart
2. The zone should read close to room temperature, within a few degrees of the other two. If it still shows an impossible value, the connection is not solid — redo it
3. Heat to printing temperature and watch the reading climb steadily, at the same pace as the other zones

Do not re-run PID tuning

Not after a sensor, not after a band, not per material. PID is never re-tuned on the G1. A sensor only needs its resistance/temperature calibration; the heater control is the factory’s — see Heater band.

Preventing it

• Support the extruder body, never the wires, when handling the machine
• At first assembly, check all three brass inserts are tight before powering on. The middle sensor is the one most often found loose after shipping
• A machine still on JST sensors will meet this again; the kit is the way to stop meeting it

Related notes

• Temperature sensor NTC 100k — what it is, where it sits, and the calibration that comes with it
• Heater not reaching temperature — when the reading is real and the heat is not
• Heater band — the heater side of the same zone
• Multi-Heating Zones
• Firmware restart