Ender 3 displaying wrong temperatures for hotend and bed

Question

I've had issues with my hot end and decided to replace the whole assembly with this. After the replacement, my printer started reading about 135/185 °C for the hot end and bed are both at room temperature. It also gives the "Heating Failed" error message. Printer halted when I tried to heat it.

Things I've tried:

1. Checked the wiring, the thermistors are reading about 110k and 10k at end of the wires.
2. Tried updating the firmware with a few different thermistor settings. Only the static (dummy) ones worked (reading a static value of 25/100 °C)
3. Tried with old thermistor and reading stayed at 135/185 °C.

I'm suspecting something wrong with the motherboard but was hoping I could figure out what's wrong before going ahead and buying a new board. Would appreciate any thoughts/suggestions.

In troubleshooting the issue, I tried disconnecting one/both of the thermistors at a time and the reading stayed unchanged. Now thinking I might've somehow broken the board when I was disassembling it.

Upon receiving a new motherboard, that fixed the bed temperature reading. The hot end is now reading -14 °C with the thermistor plugged/unplugged. Took the resistances on the old and new hot end thermistors and couldn't get reading from either. So turned out both the stock motherboard and my hot end thermistors were not working. Ordered new thermistors and that should fix the problem.

Answer 1

I got my printer fixed and am posting steps I took for people with similar problems in the future.

Initial problem:

After I replaced my entire heating unit and reconnected the wires, my printer started to read about 135/185 °C for the nozzle and bed at room temperature. It gives the "Heating failed, printer halted, please reset" error when I tried to heat the hot end and bed.

Troubleshooting steps:

1. Checked the resistance of the thermistors. Both the hot end and bed were about 100k.
2. Reflashed Marlin with different thermistor settings. Only the static (dummy) ones worked (reading a static value of 25/100 °C). All the other settings were giving very high readings at room temperature.
3. Replaced the mainboard. This fixed the bed reading but hot end was still not working. Checked the hot end thermistor again at this point and couldn't get any values, which suggests the thermistor wire was broken somewhere.
4. Replaced the hot end thermistor and that fixed all the issues.

So what I think happened was that I messed up the board when I was reconnecting the heating unit. Then at some point after I first measured the thermistor resistance, the hot end thermistor broke. Replaced the thermistor and that solved the issue.

Answer 2

For future readers,

On a second-hand 3D printer with unknown firmware, I compiled the Marlin 2.1.1 firmware for the Creality 4.2.7 board with the default configurations for the BOARD_CREALITY_V427 board, and at room temperature, the hotend reads about 121 °C.

I then measured the hotend thermistor by disconnecting the JST cable and got 9.1 kΩ with the room temperature of about 30 °C. This tells me the thermistor is a 10 kΩ NTC type.

The default configuration for Marlin is 100 kΩ NTC.

So, if you have a 10 kΩ hotend thermistor, either recompile the Marlin firmware for a 10 kΩ NTC thermistor (more trouble than it is worth if you do not know the temperature curve data), or get a new thermistor specifically listed in Configuration.h, for example:

 *  Analog Thermistors - 4.7 kΩ pullup - Normal
 *  -------
 *     1 : 100 kΩ  EPCOS - Best choice for EPCOS thermistors
 *   331 : 100 kΩ  Same as #1, but 3.3V scaled for MEGA
 *   332 : 100 kΩ  Same as #1, but 3.3V scaled for DUE
 *     2 : 200 kΩ  ATC Semitec 204GT-2
 *   202 : 200 kΩ  Copymaster 3D
 *     3 : ??? Ω   Mendel-parts thermistor
 *     4 : 10 kΩ   Generic Thermistor !! DO NOT use for a hotend - it gives bad resolution at high temp. !!
 *     5 : 100 kΩ  ATC Semitec 104GT-2/104NT-4-R025H42G - Used in ParCan, J-Head, and E3D, SliceEngineering 300 °C
 *   501 : 100 kΩ  Zonestar - Tronxy X3A
 *   502 : 100 kΩ  Zonestar - used by hot bed in Zonestar Průša P802M
 *   503 : 100 kΩ  Zonestar (Z8XM2) Heated Bed thermistor
 *   504 : 100 kΩ  Zonestar P802QR2 (Part# QWG-104F-B3950) Hotend Thermistor
 *   505 : 100 kΩ  Zonestar P802QR2 (Part# QWG-104F-3950) Bed Thermistor
...