<p>It won't show an error<sup>1</sup> if no thermistor is connected, as it will read whatever voltage is on the pin, and convert that voltage to a temperature. The pin is effectively floating, so the respective temperature could be anything.</p>
<p>That said, I seem to remember that if a thermistor had failed then, depending whether it was open or short circuit, the temperature reported would be 200°C or 0 °C (something like that)<sup>2</sup>. So, as you effectively have an open circuit, you <em>should</em> see the <em>reported</em> temperature corresponding to that.</p>
<p>From <a href="https://reprap.org/forum/read.php?4,260150,260892#msg-260892" rel="nofollow noreferrer">this post</a> on <a href="https://reprap.org/forum/read.php?4,260150" rel="nofollow noreferrer">Problem configuring my hotend thermistor</a></p>
<blockquote>
<p>The thermistor is a temperature to resistance device. The A/D converter on the Mega board is a voltage to digits device. The ramps board converts the resistance of the thermistor to a voltage by putting it in a simple bridge circuit.</p>
<p>With no thermistor attached, the bridge will apply maximum voltage to the A/D converter. That would tell the Mega that the thermistor is very cold. A very cold thermistor would mean that a lot of heat is required. There is an &quot;open thermistor&quot; setting that may or may not be enabled in your firmware. If it is, the Marlin firmware will shut things down when you unplug the thermistor.</p>
</blockquote>
<p>However, 73 °C does seem to be an odd temperature to be reported, so your Arduino Mega might have an issue. Check the A<sub>REF</sub> voltage<sup>2</sup>, which could be one possible cause.</p>
<p>Nevertheless, connect a thermistor and see if it actually works. If it does, then you can stop worrying about random edge cases, needlessly.</p>
<hr />
<h4>Footnotes</h4>
<p><sup>1</sup> You might get an error, depending on which firmware, see <a href="https://reprap.org/forum/read.php?219,775479" rel="nofollow noreferrer">RAMPS 1.4 Wierid bed temperature behaivour.</a></p>
<p><sup>2</sup> See <a href="https://reprap.org/forum/read.php?4,404221" rel="nofollow noreferrer">Thermistor reading problem!</a></p>

CC BY-SA 4.0
2024-06-22T22:11:14.987Z
Note: I have a few old Megas, RAMPS and displays lying around, so (if I get time) I'll wire a combo up and see what values I get reported (with no thermistor connected) - it will be an interesting experiment to get some hard empirical data.
Greenonline
CC BY-SA 4.0
2024-06-22T21:54:40.190Z
2024-06-22T22:00:27.257Z
2024-06-22T22:00:27.257Z
Greenonline
<p>Hot end temperature is showing without any thermistor. The attached photo is self explanatory.</p>
<p>I'm using Mega2560 (Mega WiFi R3 Atmega2560 NodeMCU ESP8266 32Mb Memory USB-TTL CH340G) + RAMPS 1.4 combination with Marlin-2.1.x.</p>
<p>I have replaced RAMPS 1.4 to 1.6 and tried again. However, still it showing temperature. Furthermore, I tried different pin i.e. 15, then it worked fine for RAMPS 1.4 &amp; 1.6. However, it is showing 58 °C / 73 °C with pin 13 &amp; 14 respectively, without any thermistor.</p>
<p><a href="https://i.sstatic.net/1Ge7kr3L.jpg" rel="nofollow noreferrer" title="Photo of RAMPS and corresponding display"><img src="https://i.sstatic.net/1Ge7kr3L.jpg" alt="Photo of RAMPS and corresponding display" title="Photo of RAMPS and corresponding display" /></a></p>
<p>I think without any sensor it should show err message however it is showing 73 °C. So the actual question is, why it is showing any temperature. Is my Mega 2560 is faulty? I have checked with with different RAMPS boards (i.e 1.4 and 1.6) also and same issue.</p>

CC BY-SA 4.0
2024-06-22T22:15:15.123Z
I'm not sure that the photo is all that self-explanatory, it is a little dark and difficult to see the board and pins correctly. Nevertheless, just to clarify a few things: (a) you have two RAMPS boards 1.4 and 1.6; (b) You connect a thermistor to A15 and it works; (c) A13 and A14 give 58°C and 73°C, without a thermistor. So... what happens when you don't connect a thermistor to A15, and what happens when you do connect a thermistor to A13 and A14?
Greenonline
<marlin><ramps>
Temperature is showing without any thermistor connected
pratik patel
# Temperature is showing without any thermistor connected

(Asked by: username_0 on 2024-06-20)

Hot end temperature is showing without any thermistor. The attached photo is self explanatory.

I'm using Mega2560 (Mega WiFi R3 Atmega2560 NodeMCU ESP8266 32Mb Memory USB-TTL CH340G) + RAMPS 1.4 combination with Marlin-2.1.x.

I have replaced RAMPS 1.4 to 1.6 and tried again. However, still it showing temperature. Furthermore, I tried different pin i.e. 15, then it worked fine for RAMPS 1.4 & 1.6. However, it is showing 58 °C / 73 °C with pin 13 & 14 respectively, without any thermistor.

[![Photo of RAMPS and corresponding display](https://i.sstatic.net/1Ge7kr3L.jpg "Photo of RAMPS and corresponding display")](https://i.sstatic.net/1Ge7kr3L.jpg "Photo of RAMPS and corresponding display")

I think without any sensor it should show err message however it is showing 73 °C. So the actual question is, why it is showing any temperature. Is my Mega 2560 is faulty? I have checked with with different RAMPS boards (i.e 1.4 and 1.6) also and same issue.

**Tags:** marlin, ramps

> **Comments:**
> **username_1**: I'm not sure that the photo is all that self-explanatory, it is a little dark and difficult to see the board and pins correctly. Nevertheless, just to clarify a few things: (a) you have two RAMPS boards 1.4 and 1.6; (b) You connect a thermistor to A15 and it works; (c) A13 and A14 give 58°C and 73°C, without a thermistor. So... what happens when you don't connect a thermistor to A15, and what happens when you do connect a thermistor to A13 and A14?  

---

## 1 answers

### Answer by username_1 on 2024-06-22. Score: 1

It won't show an error1 if no thermistor is connected, as it will read whatever voltage is on the pin, and convert that voltage to a temperature. The pin is effectively floating, so the respective temperature could be anything.

That said, I seem to remember that if a thermistor had failed then, depending whether it was open or short circuit, the temperature reported would be 200°C or 0 °C (something like that)2. So, as you effectively have an open circuit, you *should* see the *reported* temperature corresponding to that.

From [this post](https://reprap.org/forum/read.php?4,260150,260892#msg-260892) on [Problem configuring my hotend thermistor](https://reprap.org/forum/read.php?4,260150)

> The thermistor is a temperature to resistance device. The A/D converter on the Mega board is a voltage to digits device. The ramps board converts the resistance of the thermistor to a voltage by putting it in a simple bridge circuit.
>
> With no thermistor attached, the bridge will apply maximum voltage to the A/D converter. That would tell the Mega that the thermistor is very cold. A very cold thermistor would mean that a lot of heat is required. There is an "open thermistor" setting that may or may not be enabled in your firmware. If it is, the Marlin firmware will shut things down when you unplug the thermistor.

However, 73 °C does seem to be an odd temperature to be reported, so your Arduino Mega might have an issue. Check the AREF voltage2, which could be one possible cause.

Nevertheless, connect a thermistor and see if it actually works. If it does, then you can stop worrying about random edge cases, needlessly.

---

#### Footnotes

1 You might get an error, depending on which firmware, see [RAMPS 1.4 Wierid bed temperature behaivour.](https://reprap.org/forum/read.php?219,775479)

2 See [Thermistor reading problem!](https://reprap.org/forum/read.php?4,404221)

> **Comments:**
> **username_1**: Note: I have a few old Megas, RAMPS and displays lying around, so (if I get time) I'll wire a combo up and see what values I get reported (with no thermistor connected) - it will be an interesting experiment to get some hard empirical data.  

---

