I installed a new GFCI in my garage. It worked when I tested it, but once I screwed it back into the box it tripped and now remains tripped and won't reset. With my GFCI tester plugged into it, when I push the reset button, the lights that indicate it is working correctly come on and the downstream plugged in items work and then it trips after about a second. Any help?
You probably have the earth ground touching the neutral, that will cause it to behave exactly as you describe. Unlikely but it is possible the unit failed. Remove it form the box but do not unwire it and validate that again operates. The other option is something downstream is causing the problem. Unplug everything then one by one plug them back in until you find the problem.
If you have a tight fit in a box, especially a metal box, combined with wires stripped a drop more than they need to be, you can end up with a short (breaker trip) or a ground fault (GFCI trip). If you put it back in the box and it has the same problem again, take it out, remove and reattach all the wires with special attention paid to making sure that all wire under the screws is stripped but that no wire not under the screws is stripped. And if all else fails, try a wrap of electrical tape (make sure it is real electrical tape, not duct tape or "whatever I have lying around" because anything else may work now but lead to serious problems later) around all receptacle screws as you may have a "just a drop too tight" box.
So, I removed the GFCI, trimmed all of the wires up better so that there was not even one bit of exposed wire and then put a little electrical tape around the ground and over each contact before reinstalling into the box. No issues! Must've been some illicit contact :) thank you so much for the feedback, i was ready to lose my stuff the other day.