user62152: 
simulate this circuit – Schematic created using CircuitLab
Wiring is based on the scheme presented below -

(Reference: https://electronics.stackexchange.com/a/174580)
I am using Arduino Mega 2560, HX711 and four load cells to build a weighing scale but having a hard time calibrate the unit. I have tried different libraries and calibration schemes but none of them seem to help. I have found that the raw values corresponding to any given weight (including zero load) are not consistent. Firstly, the readings do not settle even after 10 minutes, and even if I take an average, the next time I put the same weight, I get an entirely different reading making the last average useless. I am seeing errors of the order of +/- 30 lbs when I put a known weight.
I first thought that maybe my load cells are the root cause but when hooked them up directly to Arduino and read the voltage across the bridge, I read stable constant values. Verified the voltage readings with a Voltmeter. (Yes, their resolution is not as good as HX711 but at least they are stable and consistent). I get the voltage reading close to my hand calculations.
Next, I tried testing the HX711 in different ways. In one test, I created a full-bridge using resistors other than load cells, forced the bridge out of balance but cannot read anything other than 16777215. Next, I wanted to see if the board is reacting to any voltage at all, so, I removed the bridge and did not connect anything to E+ and E- pins but supplied known voltages from a DC power supply (from 0 to 4.5V) to the A+ and A- (GND) pins. Now, it is reading only 8388607, no matter what voltage I supply. If I supply +5V or +3.3V from Arduino to the A+ and A- (GND) pins instead of DC Power supply, it then only reads 16777215.
I don't know how to interpret this behavior. How can I resolve the issue and bring the error down? Thanks for your help.
I have included the schematic below. All four Load Cells (LCs) are 3-wired (White, Black & Red). Resistance between W & B wires is around 1980 ohms while that between R & W or R & B is around 990 ohms. SO I connected the four LCs in the manner shown in the schematic. Wire colors have been labelled.
Code:
Code is pretty straightforward as I am simply reading raw values from the HX711 using the following HX711 library - github.com/queuetue/Q2-HX711-Arduino-Library.git 
#include <Q2HX711.h>

const byte hx711_data_pin = 4;
const byte hx711_clock_pin = 8;

Q2HX711 hx711(hx711_data_pin, hx711_clock_pin);

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  Serial.println(hx711.read());
  delay(500);
}


user61380:  
Here is is a picture of how I have seen the sensors connected.  Is this how your is?

62152: 

<p><img src="https://i.stack.imgur.com/luiUj.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fluiUj.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>

<p>Wiring is based on the scheme presented below -
<a href="https://i.stack.imgur.com/HSJFP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HSJFP.png" alt="enter image description here"></a></p>

<p>(Reference: <a href="https://electronics.stackexchange.com/a/174580">https://electronics.stackexchange.com/a/174580</a>)</p>

<p>I am using Arduino Mega 2560, HX711 and four load cells to build a weighing scale but having a hard time calibrate the unit. I have tried different libraries and calibration schemes but none of them seem to help. I have found that the raw values corresponding to any given weight (including zero load) are not consistent. Firstly, the readings do not settle even after 10 minutes, and even if I take an average, the next time I put the same weight, I get an entirely different reading making the last average useless. I am seeing errors of the order of +/- 30 lbs when I put a known weight.</p>

<p>I first thought that maybe my load cells are the root cause but when hooked them up directly to Arduino and read the voltage across the bridge, I read stable constant values. Verified the voltage readings with a Voltmeter. (Yes, their resolution is not as good as HX711 but at least they are stable and consistent). I get the voltage reading close to my hand calculations.</p>

<p>Next, I tried testing the HX711 in different ways. In one test, I created a full-bridge using resistors other than load cells, forced the bridge out of balance but cannot read anything other than 16777215. Next, I wanted to see if the board is reacting to any voltage at all, so, I removed the bridge and did not connect anything to E+ and E- pins but supplied known voltages from a DC power supply (from 0 to 4.5V) to the A+ and A- (GND) pins. Now, it is reading only 8388607, no matter what voltage I supply. If I supply +5V or +3.3V from Arduino to the A+ and A- (GND) pins instead of DC Power supply, it then only reads 16777215.</p>

<p>I don't know how to interpret this behavior. How can I resolve the issue and bring the error down? Thanks for your help.</p>

<p>I have included the schematic below. All four Load Cells (LCs) are 3-wired (White, Black &amp; Red). Resistance between W &amp; B wires is around 1980 ohms while that between R &amp; W or R &amp; B is around 990 ohms. SO I connected the four LCs in the manner shown in the schematic. Wire colors have been labelled.</p>

<p>Code:</p>

<p>Code is pretty straightforward as I am simply reading raw values from the HX711 using the following HX711 library - github.com/queuetue/Q2-HX711-Arduino-Library.git </p>

<pre><code>#include &lt;Q2HX711.h&gt;

const byte hx711_data_pin = 4;
const byte hx711_clock_pin = 8;

Q2HX711 hx711(hx711_data_pin, hx711_clock_pin);

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  Serial.println(hx711.read());
  delay(500);
}
</code></pre>
 61380: <p><a href="https://i.stack.imgur.com/33WiA.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/33WiA.jpg" alt="enter image description here"></a> </p>

<p>Here is is a picture of how I have seen the sensors connected.  Is this how your is?</p>
