What's wrong wtih my Holmes wall heater model HFH436 HFH436WGL?

I had that same problem. Some people don't understand that when you have a really small bathroom, an ordinary portable heater is in the way all winter long. A wall heater is the only practical solution. What i like about this one is the clock function which allows you to set it to automatically come on a few minutes before you wake up, so you can quickly take a shower. So I decided this was worth fixing.

First, I took the wall mount off & drilled holes in it for better air flow, since it worked better when it was just sitting on the floor. Well the holes in the wall mount made no difference, so forget that.

Well I figured out what needed to be done. The over temp limit switch is too close to the heating elements.

I bent the over temp limit switch upwards until the red wire behind it is about an 1/8" away from it. It worked like a charm. Now I can get it almost as warm as i want it. I doubt the 'stay on' function will work though, especially if you take a long shower. That's good in a way though, because you don't want it to be accidentally left alone, & melt the walls.

Take the heater and get your money back. Hopefully, you saved the box and receipt?

It might even be your electrical wiring in your house. Is it an old house? Old homes were typically not built to handle too many electronics plugged in.

Also... you can buy small heaters from Walmart for about $20 bucks that work well if not better than regular wall heaters. Should look into it. I bought a small one over 10 years ago that did wonders. I know they still sell them as seasonal items.