Red light will be on if there is +5v suppleid ok, and flash if there is less than ideal voltage to the rPi. If you have just the red light and no green light, verify your SD card first. If that checks out ok, then you just might have either a blown power diode or a bad regulator. you can replace the diode, but the regulator cannot be replaced unless you take one off of another Pi. I hear they are either programmed or serialized, so a new part won’t cut it.

That's the polyfuse. Disconnect power, and you should read close to 0 ohms across it. With power applied, you should read around +5v from either end to ground. reading an open during the ohm test tells you it needs time to reset. Same thing with reading +5 from only one side. Fuse check passes if you get close to 0 ohms and read +5 on both sides when powered up.