I have an LCD with I2C comunication and I connect it to the arduino like this:
Vcc --- 5V
SCL --- A5
When I run the I2c scanner apears : "No I2C devices found"
I can't even display the simple "hello world" example.
Can someone help? Am I doing something wrong?
Solder the Adapter properly. Use good quality jumper wires. Test them for continuity.
I2C should always have a single pair of real pullup resistors.
Many adapters have pullups. Some do not.
In practice, multiple pullups on multiple devices will "work" ok. Be careful that the effective value is between 1k and 10k.
The main lesson is to check jumper wires. Any soldered joints must be good.
You didn't say which Arduino you are using but some Arduino boards use A4/A5 and some use other pins for the I2C pins. That is why the newer boards have separate pins for SDA & SCL.
There are many potential issues with getting a LCD that uses a PCF8574 based backpack (which is what I assume you have) up and running even after reliable i2c communications has been established.
I would suggest that you use my hd44780 library package.
It contains a diagnostic tool that can help diagnose issues.
You can install using the IDE library manager.
You can read more about it here: GitHub - duinoWitchery/hd44780: Extensible hd44780 LCD library
For an i2c backpack that uses a PCF8474 i/o expander, use the hd44780_I2Cexp i/o class.
You can run the I2CexpDiag sketch to verify that the library is able to communicate with the device.
It will test the i2c signals and verify the RAM inside the LCD.
It also prints diagnostic information on the serial port.