In the diagnostics port, with the ignition off, you use a wire link to connect the TEN pin to one of the GND pins and your LED between B+ and FEN. Then you turn on the ignition (but don't start the engine) and the LED should flash to indicate any stored fault codes.
You need a 12V LED, or an ordinary LED in series with a resistor (470 ohms is a common value that's ideal). If you plug it in the wrong way round it won't do any harm but it won't light. You could check which is its + end with a 9V battery or whatever.
The flashing pattern starts with a long flash then a series of long and short flashes for each stored fault code, if any. Count long flashes for the first digit then short flashes for the second digit, then a pause before the next code. The pattern will repeat endlessly until you turn the ignition off.
The error codes can be googled but there's a nice description here: https://mx5diy.blogs...es-for-mx5.html