Can someone tell me how to fix this. I'm running it on a Serpent Cobra EP. My ESC is programmed for reverse, but when i use it on transmitter no reverse and the led blinks.

try resetting the endpoints.

I have a 3pm-x. The few times that I had that issue, I recalibrated (set the neutral, forward and brake points) the radio to the esc. (Speed Passions). Once I had to replace the sensor wire.