With what you have stated I do not believe the problem is with the rpm sensor. Usually when a garage door open will not work and the lights flash for five seconds it is because the safety sensors have a problem. I have provided you with an image below showing how to troubleshoot the safety sensors. To determine if the sensors are the problem, make sure nothing in the path of the door travel. Then push and hold the wall button in the garage. If the door goes all the way down, when you hold the button, the problem is in the safety sensors. If you follow the trouble shooting in the image below and the open still does not work I would suspect either faulty sensors or broken wires to the sensors.
If you need further assistance, please submit more details below.
Thank you for using Manage my life.
Here is a link to order any parts you need.
Sears Parts Direct
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services