I have a Tekonsha Envoy brake controller installed in my 1999 GMC 3500 dually. The light goes green when the trailer is plugged in, but I don't have any trailer brakes when I step on the brake pedal (no matter how high or low I dial it up), but I DO have brakes when I use the manual slide.  I've had this controller for a few years, and have always hated this lack of safety. It's been a running conversation with my husband for a while now. Anyway, I've finally had enough of it.

Anyone know what is going wrong with this thing? If I can't fix it quick I'm tossing it in the garbage and trying the P3 model instead, but I'm concerned I'll have the same trouble all over again and be throwing more money away.

I am not familiar with GM trucks, but Fords have a prewired plug under the dash, into which the brake controller is attached. One member of our club had problems similar to yours and it was discovered that when the plug was installed, enough force was not used to allow the two parts to "click" and lock together. After time, the two halves slightly separated. This allowed the circuit from the brake  switch to become inoperative, while maintaining 12v from the hot wire to the override function.

With the indicator light on, and the brakes working in the over ride, or manual function, I would suggest that the brake circuit from the brake switch to the controller should be checked. It sounds as if it is not getting power with the application of your truck's brakes. I would think it is not a misfunction of the controller, but a wiring problem to it.

Gard