I have a Uno R3 (label says Made in Italy) and otherwise looks like a Arduino original.
it will not put any output on A4 (SDA) or the dedicated SDA pin when using I2C to connect a LCD.
The expected clock signal on A5 (SCL) looks ok on my scope.
But A4 SDA pin is pulled LOW and even a 4.7K resistor will not pull it up
The same github test code and LCD with same I2C serial board work ok on other Uno's!
Is there a solution apart from a new Uno?
Try the blink example with an LED (and resistor) connected to A4 and replace LED_BUILTIN with A4. Disconnect every thing else from A4 to test the pin by itself. What happens?
Use a meter and check the continuity between the connector pin for A4 and Pin-27 (PC4) of the ATmega328P. I found this discontinuity problem with another connector pin of my UNO, and I corrected it by putting a jumper.
No open cct between A4 and PC4.
A simple test to drive A5-A4 HIGH/LOW as Digital Pins D19 and D18 with 1.2K R load
A5 goes to 4.9V
A4 goes to 3.7V!!!!
I also tested
A0 A1 A2 A3 and all go to 4.9V
There is a hardware issue with A4.
One more thing you can do --
Take out the MCU (if it is 28 Pin DIP) from the socket; carefully straighten Pin-27 (the PC4 pin) and put the MCU back in the socket. Activate a code to assert Logic High at this pin. Use DVM and measure the voltage level of Pin-27. If it is close to 5V, there is a leakage/loading of the PCB track on the Arduino Board. In this case, the Pin-27 can still be used; there is a spare connector pin near the IOREF pin of UNO; a jumper wire can be installed from Pin-27 to the spare pin. Good luck!