How do I fix my TX4 9,model #04041 treadmill?

I have a TX4 9,model#04041,serial#CDA 04101193 treadmill.I have replaced the sensor switch and the front roller bar due to a crack.I am now having trouble keeping the treadmill running.It will run for about 3-4 seconds and then it shuts off.The power button doesn't go off,it's just the mat stops turning.You can turn it off and start it back up and the same thing happens.The treadmill runs fine for 3-4 seconds and then suddenly stops.Can someone please help me?Your help would be greatly appreciated!