Welcome to the SeeMeCNC Community

I asked previously if anyone had written up a guide to the DEF issue and no one had. As a result, I did some searching and decided to post my result, even though they are not entirely complete and from a 3D Printing Rookie. I am not the only one experiencing this problem, but I have found it particularly annoying.

This is not a comprehensive list, but it is what I could find looking into the issue. Please feel free to contribute your knowledge where I am lacking an issue or solution, or mine was not entirely accurate (or completely wrong).

There are three most common issues that lead to the DEF display for the hotbed and extruder temperatures:

1. Physical temperature
2. PID tuning
3. Physical wiring/thermistor problems

1. Physical Temperature

I live in Michigan where it gets cold outside, which can lead to it being cold inside. If the temperature is below 18C, the machine figures there must be some kind of an issue and will not allow the printer to operate.

1. Warm up your printer
2. Change the firmware

1a. Warm up your printer
It is not good to run your printer at colder temperatures. It should be kept at a good consistent temperature. I build an enclosure to keep my printer warm. I have a small heater I can use to warm up the inside, as well as keep the area warm during printing on a particularly cold day.

1b. Change the firmware
You can simply change the firmware for the printer before you upload it. While I have used this the most, I found it is a particularly annoying method, as you need to remember to hack the firmware each time you upload a new firmware. Find "MIN_DEFECT_TEMPERATURE" in the configuration.h file and change to a lower setting, such as 12 (must be above 12C, instead of the default, which I believe is 17). See the SeeMeCNC knowledge base article. It also has other suggested solutions to the issue.

2. PID Tuning
626Pilot posted an excellent guide to Quickly hand-tune your PID values. PID tuning tells the printer how "fast" to get the bed and extruder to temperature. If the settings are significantly off, then the printer simply decides that it cannot get to temperature and will shut itself down for safety. By properly PID tuning the printer, it will reach temperature quickly, then make finer adjustment when it gets near temperature to prevent overshooting and keeps the temperature more stable. This is not the textbook definition, but hopefully you get the point.

3. Physical wiring/thermistor problems

1. Cable has fallen out
2. Failed component
3. Wiring issues

3a. Cable has fallen out
If you have the HE280, but have not printed out the whip clip. This will help hold the wiring connector in the top of the HE280.

3b. Failed component

1. bad thermistor
2. Failed heater cartridge or resistor

Xenocrates wrote:3bI. A bad thermistor will fail to read the temperature properly. The cure for this is replacement. 3bII. Failed heater cartridge or resistor. The difference in symptoms is that a bad thermistor will either DEF immediately on booting, or will stay at the current reading without change (Either too high, or low). II will only DEF after a heating attempt, as the watchdog times out. It can also show as the temperature failing to rise (Both hotend and reading in this case), or rising very slowly (due to a resistor dropping out or similar). It is very unlikely to see it rising faster than it's supposed to due to a failure, however PID can somewhat tame this (At least until a mosfet blows)

3c. Wiring issues

Xenocrates wrote:This is usually due to a short (two things connected which aren't supposed to be), or an open (Not connected when they should be). I suggest for troubleshooting this, starting by unplugging the thermistor connector, and reading the resistance between the two pins. ~100K ohms, and it usually isn't this problem. However, use a hair-dryer/heatgun/candle to warm the thermistor, to see if the resistance drops at all. If not, test from one connection point to another in series to find the source of the problem (IE, Rambo connector to HE280 connector, HE280 connector to thermistor wire, thermistor wire to thermistor body, body to other wire, etc.) . Anything other than the thermistor reading more than a few ohms is problematic. If the thermistor reads less than ~100K ohms, it is either shorted (try reinsulating the legs or resoldering), or dead. See 3b-I.

Update on 4/4/2017 to include information from Xenocrates

Posts I found related to the topic (further reading)
Error:Printer set into dry run mode until restart
Greetings from NH
Bed temp drop/DEF SOLVED!!
Temperatures read "Def"
Extruder and bed wont heat up?
Another hotend failing to reach target/upgrade?
Rostock Max v3 initial startup Thermistor problems?
Print stopped?
Problem calibrating printer
Rostock Max V3 Build with minor mods
Hot end temp issue???
First Print Great, then Blown Hotbed Thermistor
abrupt temperature failure at same postion?!
Solved - New Pre-Assembled Hot End Issues
[Solved] How Adjust Potentiometer?
LCD reset when heating hot end

How to make for a crappy start to the morning
Lost power the the LCD?
Getting this error when attempting to print with bed at 65 instead of 60
One again, getting frustrated. I turned on my v3, heated up the hot end,swapped filament, turned off the hotend, and then loaded a file and nothing happened. So, I go down to the printer, both temp readings are DEF

Re: 3b.
A bad thermistor will fail to read the temperature properly. The cure for this is still replacement. I suggest 3b become failed component, with I being a bad thermistor, and II being a failed heater cartridge or resistor. The difference in symptoms is that a bad thermistor will either DEF immediately on booting, or will stay at the current reading without change (Either too high, or low). II will only DEF after a heating attempt, as the watchdog times out. It can also show as the temperature failing to rise (Both hotend and reading in this case), or rising very slowly (due to a resistor dropping out or similar). It is very unlikely to see it rising faster than it's supposed to due to a failure, however PID can somewhat tame this (At least until a mosfet blows)

Re: 3c. This is usually due to a short (two things connected which aren't supposed to be), or an open (Not connected when they should be). I suggest for troubleshooting this, starting by unplugging the thermistor connector, and reading the resistance between the two pins. ~100K ohms, and it usually isn't this problem. However, use a hair-dryer/heatgun/candle to warm the thermistor, to see if the resistance drops at all. If not, test from one connection point to another in series to find the source of the problem (IE, Rambo connector to HE280 connector, HE280 connector to thermistor wire, thermistor wire to thermistor body, body to other wire, etc.) . Anything other than the thermistor reading more than a few ohms is problematic. If the thermistor reads less than ~100K ohms, it is either shorted (try reinsulating the legs or resoldering), or dead. See 3b-I.

When I plug in I2C to the RAMBO (version 1.1b) -it was printing and working, but during the print, the RAMBO crashed and I could not get my RAMBO to boot any further than the FIRST LCD SCREEN! After several hours, trying to clear the Firmware, NOTHING WORKED, so I started unplugging wires from the Rambo, and as soon I took I2C out, I was able to clear the Firmware and re-upload the new version! As soon I put I2C back to the I2C on the Rambo and turn the printer on, I get RED LED - it is called LED2, it's to the right of the LED1 that has under it LED7. AND the LCD screen can't boot any further than the FIRST SCREEN! Does anyone out there knows what the heck is going on and how to fix it? Thanks.

I have little more info - just copied it from the Facebook SeeMeCNC:
I WAS HAPPY but it did NOT LAST!
During a print, LCD crashed and I could not boot the printer LCD -ONLY the first screen and it never went any further! After few hours of RED LED on the RAMBO, nothing I was able to find here or anywhere - how to fix this, I started unplugging some wires. As soon I unplugged the I2C I was able to CLEAR Firmware and REINSTALL my Firmware! Boot fine, but as soon I plugged in the I2C I had more problems. Thanks GOOD I've purchased a backup HE280, but that did not fix all the problems, because I CAN'T PRINT ANYWAY - I'm getting an error in MatterControl - that it is in DRY RUN, must reboot... BUT id DIN'T WORK! Shutting everything down, rebooting PC and my printer several times .... the same error is there! I'm just at the end of my rope!

I WAS HAPPY but it did NOT LAST!
During a print, LCD crashed and I could not boot the printer LCD -ONLY the first screen and it never went any further! After few hours of RED LED on the RAMBO, nothing I was able to find here or anywhere - how to fix this, I started unplugging some wires. As soon I unplugged the I2C I was able to CLEAR Firmware and REINSTALL my Firmware! Boot fine, but as soon I plugged in the I2C I had more problems. Thanks GOOD I've purchased a backup HE280, but that did not fix all the problems, because I CAN'T PRINT ANYWAY - I'm getting an error in MatterControl - that it is in DRY RUN, must reboot... BUT id DIN'T WORK! Shutting everything down, rebooting PC and my printer several times .... the same error is there! I'm just at the end of my rope!

I would like to share a couple of additional failures' root causes, related to the def and causing mid-print stops, that I have recently experienced.

1- The first is related to the Kapton tape around the thermal fuse.
If the fuse body is not properly covered with the tape then it can short with the heat sink. Worst than this, when assembling the hot end you might force the fuse into its housing damaging, without knowing, the proper tape protection. On the long run, hot-end vibrations and other environmental factors can further degrade this installation shorting the fuse. In my case, the short was occurring after few hours into the print, due certainly to the heat sink dilatation and hot end vibration, exposing unprotected fuse parts to contact the heat sink (Fig. 1). The remedy was to re-do the fuse protection. I would recommend adding a warning sign to highlight this aspect in the HE280 upgrade instructions so that builder is made aware of this critical aspect.

2- The second defect is located on the HE280 electronic board.
There again, vibrations have induced a board surface wear nearby one of the fixing screws causing the PS-ON circuit to short with the screw itself (Blue wiring, Item 8, Fig.2). In this case, the remedy was to apply the Kapton tape to isolate the damaged circuit board line from the screw (Fig.3 & 4).

Francesco