I2C Scanner stuck on Scanning ...

Board: Pro Mini 5V (third party)
oLED: Here

Attempting to connect oLED to Arduino. Running I2C scan and it’s just stuck on scanning if the board is connected. If the board is not connected, it says, ‘No I2C devices found’

I’m using SDA to A4, SCL to A5. There are two 10k resistors in the SDA, SCL path - pullup resistors? Not sure. I attempted with/without with the same outcome. I’ve tried soldering for better connections, same result. It will sometimes output ‘Unknown error at address 0x01’, 2, 3 etc. but freezes after a few (random ranges).

I’m very new to arduino. Any help is appreciated. I’ve wasted hours trying to figure this out, getting very frustrated.

Photos attached.


Running I2C scan and it's just stuck on scanning if the board is connected. If the board is not connected, it says, 'No I2C devices found'

This is a typical symptom of SDA/SCL shorted to each other or ground. Look very carefully for any solder bridges or other possible problems

The OLED displays work with 3.3V. This one seems to be compatible with 5V, but I don’t know how reliable that information is.

A resistor in the signal path will not work.
I think you have a resistor in the same row of the breadboard, the pins of that row are already connected to each other.

Can you connect a 10k resistor to SDA and 5V.
And also a 10k resistor to SCL and 5V.
When the I2C bus is not active, then those resistors will pull the signal to 5V. That’s why they are called pullup resistors.

I have my doubts with the soldering of the pins A4 and A5 on the Pro Mini board.
There are clone Nano boards with a CH340G usb-serial chip that have the A4 and A5 along the side, and A6 and A7 somewhere in the board.

I don’t use Pro Mini boards anymore. When I want to change a sketch, then I have to search for the usb-serial board. A small Arduino board with a usb connector is easier.


I have checked all connections carefully. I see no bridges or anything weird. I've also checked under the LCD screen to make sure it's not grounding anything.


Alright - just want to make sure this is what you're saying.

From oLED SDA pin, there should be a 10k resistor to a 5v power source and in the same row a connection to A4 on mini, from SCL pin the same and to A5. If this is the setup, I'm getting the same results. I've tried powering both with 3.3V and 5V.

Note* I've also tried different cables. I also believe this is a clone board, which is why A4/A5 are in the center instead of edge of the board.

Yes, the OLED SDA to Arduino SDA and a pullup resistor from SDA to 5V.
The same for SCL.

In the past, some OLED displays never activated the SDA signal. They were not able to send a acknowledge to the Arduino and a i2c scanner did not work. Your display gets 4.8 stars out of 5, that is really good. The display should work normally.

A wire could be broken, or the breadboard could be bad.
Can you try with other wires on an other location of the breadboard ?

If that does not work, then I suggest to use a different Arduino board, a different I2C device, an other breadboard, and other wires. Perhaps even an other computer and another USB cable.
For example a I2C EEPROM or I2C clock module (RTC). Connect that to a Arduino Uno and run a i2c scanner.
If that works, then you can exchange things until the faulty device is found.

What else with I2C do you have ?
A logic analyzer could help.