Many issues that people have with openinverter can be solved with basic electronics knowledge. Since many folks who convert EVs rather have a mechanical background this is often a shortcoming.
This page aims to be a quick guide on basic electronic engineering practises that let you diagnose and fix most issues.
More specialized FAQs are: Inverter FAQ and Tesla Setup FAQ
No LEDs light up on the board
All boards that you buy either in the openinverter shop or at EVBMW have at least been powered up once to program them. So unless damage was done during transit they are an unlikely culprit.
You are having a problem with the supply voltage. On most openinverter boards there are 3 voltages: 12V, 5V and 3.3V. Start at the very feed-in point of the voltage, then work your way down (TODO: add pictures here). Attach your multimeter black lead to a solid ground point (the negative supply voltage) and then work with the red lead.
Now you may find that 12V do not even arrive on the board. That is easy, check for bad solder joints, loose crimps or broken cables.
You may find that the 12V supply arrives ok but then the 5V supply is too low or 0. Since the boards have undergone testing this is likely caused by something external. The boards usually also supply external circuitry such as gate drivers, current sensors etc. with supply voltage. If one of these external circuits is broken, it can short out or overload the power rail. This results in onboard LEDs being off or dim. Note that lower voltage supplies are derived from the higher voltage ones. So if 5V is overloaded, 3.3V will also not receive (sufficient) power.
So disconnect ALL external circuitry and only supply 12V to the board. Then plug things in bit by bit to find the culprit.