When I rewired my house 5 years ago I installed a GFCI outlet in the main bathroom. On the same line, after the GFCI, is another outlet in that bathroom, a junction box in the attic, an outlet in my small bathroom and then the wall switch to control the lights and fan in the small bathroom.

Through the years I would run into a problem every now and then. It seemed if I turned on the light and fan in the small bathroom at the same time or in quick succession, the GFCI would trip. I always assumed it was a faulty wall switch until last month when the GFCI will not reset.

I tried replacing the GFCI, twice, but that didn't solve the problem. In trying to work my way back through the line I snaked through new lines from the junction box into the small bathroom today... still no luck. The breaker in the panel doesn't trip, just the GFCI. When I bypass the GFCI everything works fine and the breaker still doesn't trip. I'm guessing there has to be a small amount of electrical leakage/drain somewhere in the line but when I plug in an outlet tester it reads that everything should be fine.

Before I start opening up the walls in my main bathroom, does anyone have any advice?

Thanks,
Logan

You have lights, outlets and a fan on this circuit all downstream of the GFCI...
Try removing individual items from the circuit and reset the GFCI, see which one is the culprit...
My nickel is on the fan!.

If this doesn't reap positive result, I would consider taking all the downstream devices off the load side of the GFCI... none of them are required to be protected except for the receptacle in the small bathroom which you will need to change to a GFCI...

The amount of ground fault current to trip a GFCI is .005 amperes (5 mA) a very small amount. Sometime long conductor lengths that are GFCI protected will cause tripping also...

Good Luck

It's not the fan but I like what you said about pulling the other items off the line if I rewire. I started at the end of the line and started pulling things off as I worked my way upstream. I pulled everything off and it still tripped. Eventually I disconnected the incoming wire from the junction box that's in between the two bathrooms and wire nutted the end of it to an extension cord with a female end... still no luck. So all I had left was the GFCI, the wire leading out, and the extension cord attached to the end. I tested the line and it seemed fine until I plugged in a lamp.

Now all I have downstream of the GFCI is a wire. This is what is making me believe that I have to open the wall in the bathroom. Yes?

Tell me, if you disconnect the "load" conductors off the GFCI, does it trip?

As I tried to suggest, remove everything from the "load" side conductors of the GFCI... then
Install a new GFCI at the second bathroom. Avoid putting anything on the load side of that GFCI also...