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  • I've actually used one of these with an MDBT42 so I know it works... There is no need for resistors (at least when I did it) - and as @AkosLukacs says the MDBT42Q isn't 5v tolerant so pulling up to 5v won't be good for the IO pins anyway.

    Stuff you could try though:

    • Is the LCD definitely running on 5v?
    • All the GND pins should be connected together.
    • Are you sure the I2C pins are the right way around? Maybe try swapping them just in case - it won't break anything if they're wrong.
    • Try var i2c = new I2C(); i2c.setup(...); require("HD44780").connectI2C(i2c); to use software I2c, just in case.
    • If all that fails, could you send us a picture of the back of the LCD? It's possible it's using a different IO expander to the usual ones.

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