Nozzle temp always reads 236 °C, even when it's room temp (Ender3 V2 Neo)

Question

My first 3D printer, an Ender 3 V2 Neo. The original issue I was working to address was a clogged nozzle. The heat-block was caked in melted filament. At this point, I just chalked it up to a gap forming between the Bowden tube and the nozzle. I decided it was enough to replace the nozzle and the stock tubing. However, in my vain attempts to replace the nozzle while it was at room temperature, I had gone to town holding the heat-block in my left hand with pliers, and the nozzle with the included cone wrench with my right.

After learning why it's necessary to risk burning your fingers on a 240 °C nozzle while performing maintenance I was able to replace the nozzle. However, the problem carried over to nozzle #2. After inspecting conditions I thought the issue at this point was a gap between the heat-break and the heat-block/nozzle that formed due to my earlier "repairs".

I disassembled everything, removing the heat break from the heat sink, the heat block from the heat break, and cleaned them as best as I could. (My tom-foolery included removing the heating element from the heat block as I didn't know what a grub-screw was, and then experiencing how exciting it is when the heating element slips out of the heat block)

After putting everything back together again, the display began showing the current nozzle temperature at 236 °C. As in 236 °C is displayed regardless of whether I just turned the printer on, or if I manually set the temperature to 200 °C. The nozzle doesn't heat up and it's safe to touch. If I manually set the nozzle temp to 250 °C and wait, the display will show the nozzle heating up to 250 °C, however, the nozzle's no warmer than a sunny day.

While the printer sits idle I do periodically see temperature fluctuations of plus/minus 1 °C.

Being unable to fix the problem by working on the hotend I assumed something happened to the thermistor so I purchased an entirely new (stock) hotend.

After installing it I flipped the power switch and the nozzle was still at 236 °C.

I tried to replace the firmware, first by installing 1.1.5.2, I verified it installed through the info screen but there was no change in the temperature reading. I then installed the factory version, 1.1.4 again to no effect. (Creality's Downloads page)

All that's left is to replace the entire board which feels excessive for such a seemingly tiny issue. Am I missing something? I'm not able to find anything on Google that resembles my problem.

Answer 1

If the temperature reading shows increases above 236 when heating, but can't show anything below, you have a short of approximately 300 ohms across the thermistor wires. This could be something wrong on the board or in your wiring to the hotend. You could try diagnosing this by measuring the resistance across the thermistor connector on the board. If you see somewhere around 300 ohms, the problem is on the board and unless you can find and fix the cause, you probably need to replace the board.

As an aside, if the board did not kick into thermal runaway protection during the entire time it took to heat from room temperature to over 235, during which no change in the read-back temperature occurred, the firmware has missing or broken thermal runaway protection and is unsafe to operate (severe fire hazard). You should check on this even if you get it working again.

Answer 2

I really suspect a defective or disconnected thermistor. Can you measure the voltage across the thermistor at the connector on the board when turned on? The value should change when you heat the hotend (use a hair dryer).

If the value does change, the error is in the firmware / board. If not, recheck your connections for continuity with a multimeter.