What is an MQ5 sensor

MQ 5 is for propane butane or LPG for liquid gas. The sensor also reacts to alcohol, carbon monoxide, natural gas and smoke gases, but for this
There are other sensors for gases; there are others.

Simply connect as shown in the picture. GND, + 5V and A0. I haven't found out what D0 is for yet.
The red LED should light up, the green one can. The potentiometer is used for calibration. Simply stretch until the green LED goes out, then you have the threshold point.
Test with a gas lighter without a flame. The green LED should now light up

In the serial output of the Arduino you can see a clear increase in the analog value.
For an exact display in PPM, a reference measurement with a known quantity must be made.
I haven't figured out how to do that yet. But you can tell that something is there and if something is there then something is leaking. That should be enough to check various lines and hoses for leaks.
You could change the sketch a bit and let a buzzer beep when the AD value rises. Great warning for caravan owners.

Here is a simple sketch to test.

MQ7 CO sensor

The breakout is constructed in the same way as that for the MQ5, only the sensor is specially designed for the dangerous carbon monoxide.

The sketch is similar to that of the MQ5 but also evaluates the digital value, which changes to high at the threshold value.

/ * MQ-7 Carbon Monoxide Sensor Circuit with Arduino * /

const int AOUTpin = 0; // the AOUT pin of the CO sensor goes into analog pin A0 of the arduino
const int DOUTpin = 8; // the DOUT pin of the CO sensor goes into digital pin D8 of the arduino
const int ledPin = 13; // the anode of the LED connects to digital pin D13 of the arduino

int limit;
int value;

void setup () {
Serial.begin (9600); // sets the baud rate
pinMode (DOUTpin, INPUT); // sets the pin as an input to the arduino
pinMode (ledPin, OUTPUT); // sets the pin as an output of the arduino
}

void loop ()
{
value = analogRead (AOUTpin); // reads the analog value from the CO sensor's AOUT pin
limit = digitalRead (DOUTpin); // reads the digital value from the CO sensor's DOUT pin
Serial.print ("CO value:");
Serial.println (value); // prints the CO value
Serial.print ("Limit:");
Serial.print (limit); // prints the limit reached as either LOW or HIGH (above or underneath)
delay (100);
if (limit == HIGH) {
digitalWrite (ledPin, HIGH); // if limit has been reached, LED turns on as status indicator
}
else {
digitalWrite (ledPin, LOW); // if threshold not reached, LED remains off
}
}