Nozzle temperature fluctuating: why your hotend won't hold its temperature

This guide is for anyone whose nozzle temperature drifts, swings, or drops during a print, or whose printer stops with a hotend temperature error. It explains what the readout is telling you, the causes Prusa, Bambu Lab, Creality, E3D, and the Marlin and Klipper firmware projects document, how to tell a tuning problem from a hardware fault, and how these problems show up in prints.

How hotend temperature control works

A hotend has two parts that matter here. A heater (a cartridge on many hotends, a ceramic heater on Bambu Lab and Creality K1 hotends) makes the heat. A temperature sensor, usually a thermistor (Bambu Lab calls it the NTC), reports it. The firmware compares the reading to the target and adjusts the heater's power.

Marlin, Klipper, and Prusa's firmware all use PID control for this. Marlin says that when PID values are set correctly, "heaters reach their target temperatures faster, maintain temperature better, and experience less wear over time." Prusa describes PID as what is used "to maintain a stable temperature on the hotend."

This means a fluctuating readout can come from three places: the control settings, the heat being lost faster than the heater can replace it, or a sensor or connection that reports the wrong number.

What makes the nozzle temperature swing?

PID settings that don't suit your printer or room

Creality's Ender-3 V3 SE troubleshooting page says large temperature fluctuations can happen when "the pre-set PID temperature of the machine is not suitable for the local environmental temperature." Klipper's documentation says PID settings "found in other firmwares or in the example configuration files often work poorly," so each printer needs its own calibration. Marlin says PID should be retuned "whenever changing a hotend, temperature sensor, heating element, board, power supply voltage (12/24V), or anything else related to the high-voltage circuitry."

Prusa sets a limit on what tuning can fix. Its PID article for the MK2.5 and MK3-family printers says tuning is for "small fluctuations" such as "+/- 5 °C." Swings larger than that suggest "a problem with your fan or thermistor wire/connector." Creality's Ender-3 V3 SE page takes a different starting point: for fluctuations it calls significant, where the hotend still reaches its set temperature, its first step is to run the automatic PID. Follow the guide for your printer.

The part-cooling fan blowing on the heater block

The fan that cools your print can also cool the hotend. Prusa says a damaged or tilted fan shroud "can change the airflow and start cooling the heater block directly," and that "for safety reasons, the heater doesn't have enough power (by design) to compensate for that." Marlin lists "a fan blowing on the heat block" among the things that slow heating.

Some drop is expected. Prusa says that on its i3-series printers, a drop of 5 °C to 10 °C when the print fan switches on, "especially when the printer transitions from the 1st to the 2nd layer," is normal, and "the nozzle temperature should recover rather quickly as the PID regulation kicks in." The problem is a drop that doesn't recover. For the 12-volt MK2.5/S, Prusa says you may need to run the print fan at only 70 to 80% when printing at PLA temperatures.

A missing, loose, or damaged silicone sock

Bambu Lab says the silicone sock "helps maintain nozzle temperature stability by insulating the hotend." It lists two signs of a missing or damaged sock: an HMS error for "abnormality in the nozzle temperature control," and "temperature fluctuations," where "the set temperature might not match the measured temperature and fluctuates inconsistently." A loose sock lets in "cold air from the environment," which Bambu says "may cause temperature fluctuations."

Bambu says to replace the sock if it has cracks or tears, has become loose, has stretched or deformed from heat, is discolored or brittle, or rubs against moving parts such as the nozzle wiper. Frequent filament build-up on the heater block can also mean the sock has failed.

Not every maker fits one. E3D supplies a silicone sock with its V6 hotend and lists the V6's maximum operating temperature with the sock fitted as 260 °C. Prusa says that in its testing of the MK2.5/S it "didn't find the E3D sock to be very beneficial, as it can fall off and cause print failures and even damage the printer," though it may be worth trying if thermal runaways continue with the fan at 70 to 80%. If your printer shipped with a sock, keep it on and in good condition. If it didn't, check your maker's guidance before adding one.

A loose thermistor, heater, or cable

If the sensor isn't firmly in the heater block, it reads the wrong temperature. Prusa says the thermistor cartridge on its i3 printers should sit fully inside the block, "with no part of it sticking out." On the CORE One, MK4, and MK3.9 families, it says to make sure the thermistor is "fully inserted in the heater block, and not loose."

Damaged or loose wiring causes the same kind of trouble. Creality's K1-series guide lists a thermistor wire that isn't fully inserted, a loose ceramic heater or thermistor wire, cracked solder joints on the heater wires, and a loose thermistor as causes of its 2564 "extruder not heating as expected" error. Bambu Lab lists "poor contact between cable and connector" and a broken sensor wire among causes of its sensor open-circuit error. Marlin says its "Thermal runaway" error "may indicate poor contact between thermistor and hot end, poor PID tuning, or a cold environment."

Prusa's thermal model article for the MK3/S/+ adds other assembly problems: a hotend that isn't seated correctly, a nozzle that isn't tightened with the correct gap to the heater block, and "filament residue on the heaterblock," which it calls "a cause for thermal anomalies."

A cold room, drafts, or a weak supply

The heater has a fixed amount of power to work with. Prusa's PID article says to keep the printer "in a dry environment with a stable temperature over 20 °C and no breeze." Its i3 thermal runaway article suggests an ambient temperature of 16 °C or more, and warns that "an unheated garage in winter might cause thermal runaway as the heater doesn't have enough power once the print cooling fan kicks in." For the CORE One, MK4, and MK3.9 families, Prusa says the room should be "stable, and above 18 ºC." For heated beds, Prusa warns against placing the printer near an air-conditioning unit or an open window.

Creality's Ender-3 V3 SE page lists "insufficient heating power" as the second cause of large fluctuations, and suggests measuring the voltage at the mainboard's hotend port while heating if you have a multimeter. Marlin says heating may be slowed "if the heater has high resistance."

How does it show up in prints?

• Ridges or bands up the side of the print. Simplify3D says that when a poorly tuned PID controller lets the extruder temperature cycle, the plastic flows differently from layer to layer, "creating visible ridges on the sides of your print." Rule out the mechanical causes in our Z-banding guide too, because they look similar.
• Thin or missing extrusion. Polymaker says printing too cold "can cause too little of filament to extrude," and that a cooling fan blowing on the heater block and nozzle can drop the extrusion temperature in the middle of a print. See our under-extrusion guide.
• Clogs and jams. Bambu Lab says that when the heater is short-circuited, "the nozzle cannot heat up," the hotend can't extrude, and "the filament may be jammed in the heatbreak." If your temperature is steady but prints still stop extruding partway through, see heat creep and our stopped extruding mid-print guide instead.
• Plastic building up on the heater block. Bambu Lab links frequent build-up around the hotend to a failed sock. Prusa says filament residue on the heater block is itself "a cause for thermal anomalies," so clean off build-up rather than ignoring it. See our nozzle leaking guide.
• A print that stops with an error. When a printer's thermal protection trips, it turns off the heaters and the print ends. The errors are covered below.

How do you diagnose it?

Watch the temperature reading

Watch the nozzle temperature during a real print, not just while it preheats. Klipper's setup guide uses the temperature graph in the printer's user interface. On Prusa's MK3S/MK3/MK2.5S/MK2.5, Prusa says you can see readings from all thermistors under Support > Temperatures on the LCD menu.

Note what the swing looks like:

Try Prusa's cable test

For its i3 printers, Prusa suggests moving the extruder left and right along the full length of the X axis, and wiggling the cable bundle behind it, while watching the temperature. Fluctuations during this test "would indicate an issue with the cable." Do this only at room temperature or while watching from a safe distance, as your printer's guide allows. Don't put your hands near a hot nozzle.

Look at the hotend with the printer off and cold

With the printer powered off, unplugged, and cooled, check that:

• The silicone sock is present, intact, and snug, if your printer uses one (Bambu Lab).
• The part-cooling fan shroud isn't bent, cracked, or tilted toward the heater block (Prusa).
• The thermistor is fully inserted in the heater block (Prusa).
• There's no plastic crusted around the heater block (Prusa, Bambu Lab).
• The thermistor and heater connectors are fully seated, and the cables aren't damaged along their length (Prusa, Bambu Lab, Creality).

On some printers you have to remove covers or the sock to see these. Prusa says to remove the silicone sock, if installed, to reach the set screws that hold the thermistor and heater on the CORE One, MK4, and MK3.9 families. Use your maker's guide for that step.

Measure with a multimeter, if your maker gives values

Several makers publish room-temperature resistance values for checking a heater and thermistor with the printer off and cold. They differ by printer, so only compare against the figure for your exact model:

E3D says to check its V6 heater cartridge with a multimeter before installing it, and that if it reads out of specification, "do not attempt to heat up the heater cartridge." Prusa says that if a thermistor or its cable is damaged, it "must be replaced completely," because splicing or soldering can change the resistance the mainboard reads.

Fixes by printer and firmware

Prusa

For the MK2.5, MK2.5S, MK3, MK3S, and MK3S+, Prusa says PID tuning is in LCD Menu > Calibration > PID calibration, starting with firmware 2.0.12. Set the temperature you print at most often. Prusa says the nozzle heats in 5 cycles while the printer learns how much power it needs, and that one calibration "will increase the stability for all temperatures." Prusa is clear that tuning "is not a solution for every temperature fluctuation issue."

The MK3/S/+ also has thermal model protection from firmware 3.12.0. Prusa says the printer compares the measured temperature to a model of what it expects. If they disagree, it beeps and shows THERMAL ANOMALY but keeps printing, and if the anomaly continues, it turns off the heating and shows an error. Prusa says this helps with temporary problems "like when a window is opened near the printer," and can stop a print when a blob wraps around the hotend. If the warning keeps appearing, Prusa's steps are a visual inspection, then a PID tuning followed by a new Temp Model calibration under Calibration > Temp Model cal., then a multimeter check. If you use an enclosure, Prusa says the Einsy board needs to be inside it, or the thermal model can trigger falsely.

We didn't find a user PID calibration in Prusa's articles for the CORE One, MK4, or MK3.9 families. For those, follow Prusa's thermal runaway checks below.

Bambu Lab

We didn't find a user PID calibration in Bambu Lab's wiki. Its guidance for nozzle temperature problems focuses on hardware: check the silicone sock first, then reseat the NTC (thermistor) cable, check the hotend and its wires, and measure resistance. Bambu says the "quick check" for its heater over-temperature error on the X1, P1, and A1 series is to try a complete new hotend and see if the error clears. If the message keeps showing, it says to contact its after-sales team.

Creality

On the Ender-3 V3 SE, Creality says to run the automatic hotend and bed PID under Control > Temperature > Auto PID. Creality also has a general guide to PID tuning by sending G-code from Simplify3D's control panel. It doesn't name the printer models it applies to, and it refers to Marlin's Configuration.h, so it's for Marlin-based machines. It uses M303 C8 S200 for the hotend, which it says takes about 6 minutes, then M301 with the P, I, and D values your test reports, then M500 to store them. Creality says PID values "are different according to the heating block, heating power, heating voltage and external environment," so use the values your own test produces, not the example values in its guide. We didn't find this procedure on Creality's pages for the K1 series, so check the wiki page for your exact printer before sending commands.

Marlin firmware

Marlin's PID autotune command is M303. Its documentation gives the form M303 [C<count>] [D] [E<index>] [S<temp>] [U<bool>], where C is the number of cycles (at least 3, default 5), E is the hotend index, and S is the target temperature. Marlin's example M303 E0 C8 S210 tunes the first hotend at 210 °C for 8 cycles, and adding U1 applies the result. With EEPROM support enabled, PID values are saved with M500, and M503 shows the current values.

Marlin's advice on the target differs from Prusa's. Marlin says to "use the highest target temperature you intend to use (where overshoots are more critical)." Prusa says to use the temperature you print at most often. Follow the documentation for your printer's firmware.

Klipper firmware

Klipper's command is PID_CALIBRATE. Its documentation gives the example PID_CALIBRATE HEATER=extruder TARGET=170, then says to run SAVE_CONFIG at the end of the test to write the new PID settings to printer.cfg. Klipper says the heater is enabled until it reaches the target, then "turned off and on for several cycles." If your printer runs a maker's customized Klipper, check the maker's guide before editing configuration.

Any printer

• Keep the sock on. If your hotend came with a silicone sock, keep it fitted and replace it when it cracks, loosens, or deforms.
• Check the fan duct. Make sure the part-cooling duct points at the print, not at the heater block. Replace a bent or cracked shroud with your maker's part.
• Move the printer out of drafts. Keep it away from open windows, air-conditioning vents, and cold rooms, as Prusa advises.
• Retune after hardware changes. Run your firmware's PID calibration after changing the hotend, heater, or thermistor, as Marlin advises, if your printer offers one.
• Clear blobs early. If plastic wraps around the hotend, follow your maker's cleaning guide. Bambu Lab lists severe blobs, and removing them the wrong way, as causes of a hotend that can't reach its set temperature, because they "may damage heating cables."

Thermal runaway and heating errors

These errors are safety shutdowns. Prusa says thermal runaway exists because if a thermistor came loose, the printer would read a falsely low temperature and the heater "could reach dangerously high temperatures, with the printer potentially becoming a fire hazard." Treat them as a reason to inspect the printer, not as a nuisance to silence.

What each maker documents:

• Marlin. Marlin describes two levels of protection. If the temperature fails to rise enough while heating (by default, 2 degrees in 20 seconds), the machine shuts down with a "Heating failed" error, which "will detect a disconnected, loose, or mis-configured thermistor, or a disconnected heater." If the temperature drifts too far from the target for too long, it shuts down with "Thermal runaway." Marlin says Err: MAXTEMP "usually means that the temperature sensor wires are shorted together," and may also mean the heater's MOSFET or relay is stuck on.
• Klipper. Klipper checks every heater automatically. Its verify_heater documentation says the temperature is inspected once a second, and if it stays below the target range, an internal error counter grows until an error is raised. Klipper's FAQ says the microcontroller also shuts down all heaters and steppers if it detects a temperature outside the configured minimum and maximum, or if it stops hearing from the host.
• Prusa i3 series. Prusa says Thermal Runaway shuts down the printer "when the temperature drops by more than 15 °C for more than 45 seconds."
• Prusa CORE One, MK4, and MK3.9 families. Prusa says the "Hotend thermal runaway" error shows when the nozzle drops by 12 °C and the heat isn't recovered after 60 seconds. Its checks are a stable room above 18 °C, a fully inserted thermistor, undamaged heater and thermistor cables, the extruder fuse on the xBuddy board, and the main cable from the print head.
• Bambu Lab. Its HMS codes in the 0300-0200-0001 range cover nozzle temperature faults. Bambu says its "heater over temperature" error is raised when the heater runs at full power but can't bring the nozzle to the target in time, and lists a broken or badly fitted sock, a poorly installed or faulty heater or NTC, poor connector contact, and a faulty TH board as causes. Its sensor open-circuit error, which Bambu says can make the heater "heat continuously," starts with a check of the silicone sock.
• Creality. Creality's K1-series 2564 error reads "extruder not heating as expected" and points to loose or damaged heater and thermistor wiring, cracked heater solder joints, and a loose thermistor. Its TE2509 error, "Unable to obtain nozzle temperature," means the thermistor is open-circuited. Creality says that if the wiring is fine "but the screen reading is negative," the thermistor or hotend needs replacing.

Marlin's documentation includes build-time settings that change how sensitive these checks are. They're meant for people who build their own firmware, and they don't fix a loose sensor. On a stock printer, work through your maker's troubleshooting steps and contact its support if the error keeps coming back.

A sensible order

1. Watch the temperature during a print and note how big the swings are and when they happen.
2. If the swings are small and steady, run your printer's PID calibration, where it offers one.
3. With the printer off and cold, check the silicone sock, the fan shroud, and for plastic on the heater block.
4. Move the printer away from drafts and out of cold rooms.
5. If the swings are large or the reading jumps when the head moves, check the thermistor and heater connections using your maker's guide.
6. If your maker gives resistance values, measure the heater and thermistor with the printer off and cold.
7. If you see thermal runaway or heating errors, follow your maker's steps for that exact error, and contact support if it continues.

When not to buy parts

Hold off on a new hotend, heater, or thermistor if:

• The swing is small and steady. Try PID calibration first, where your printer has it.
• The sock is missing or torn. A replacement sock from your printer maker is the cheap first fix on printers that use one.
• The dip only happens when the fan starts, and it recovers. Prusa says that's normal on its i3 printers.
• The printer sits by a window or vent. Move it and test again.

Replacement parts make sense when your maker's checks point to them, such as a resistance reading out of range, a damaged cable, or an error that continues after reseating the connectors. Bambu Lab suggests trying a complete new hotend for its heater over-temperature error, and Creality says a negative reading with good wiring means the thermistor or hotend needs replacing. Buy parts that match your exact printer. Our replacement nozzle picks list maker-documented fits for several printers.

How this guide was checked

This is a research-based guide. We haven't run temperature tests for it. Every command, error name, temperature, time, and resistance value comes from the Prusa Research, Bambu Lab, Creality, E3D, Marlin, Klipper, Simplify3D, and Polymaker pages listed above, retrieved on 2026-10-14. Values differ between printers and firmware versions, so check your own maker's current documentation before relying on a figure. Read more in our editorial standards.

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