Why t3xi treadmill is stopping while someone is running

E1 usually suggests it is and Electrical error and would probably require someone with knowledge of treadmills to rectify the error.
It is very unlikely the ordinary Jo Blo can fix this problem.

If you wish to get some details; check the site linked here. Pull up older posts...
http://electro-medical.blogspot.com/

Chances are the treadmill started by shutting off in the middle of a workout and has gradually gotten to the point that it will not run long, or turn on at all, in some cases. This is most likely caused from a worn deck and belt. If the belt and deck is not routinely waxed, it wears down and causes friction. This friction builds up heat which can fry your electronics. The machines circuit breaker shuts the machine down to protect the major electronic components. This protective measure, built in by the manufacturer, will only help for so long. After a few times of shutting down due to the heat, it actually does damage to the electronics. The result and correction usually ends up being a new deck, a new running belt, and in most cases that have gone on too long, the motor control board. I am attaching a link for the service manual for further information. Parts for Matrix T3 Treadmills T3x T3xi Belts Rollers Decks SPORTSMITH...

https://support.johnsonfit.com/resource.php?url=http://productload.johnsonfit.com/inc/uploaded_media/5cef253ea328319b0586204baaac6bf6/owners_guide/3f10af8d9932e71dee9fd0299c13b634.pdf

You can find parts for this model at the following link.
Parts for Matrix T3 Treadmills T3x T3xi Belts Rollers Decks SPORTSMITH...

see the diagram attached for the speed sensor. test for open or faulty. clean it, and tight for plug or connector.
2) Test the input voltage motor, if there are the motor need repair
3) Test the output voltage board, for input voltage motor, if there are NOT the board is faulty.
God bless you

tight the ribbon cable keypad, clean it +tight the wires connector from display board vs lower board and clean it.
2) adjust the reed switch, clean it and tight the wires connector.

see the diagram.

God bless you

tight the ribbon cable from keypads , clean it +tight the wires connector from display board vs lower board and clean it.

2) adjust the reed switch, clean it and tight the wires connector too, see the diagram

God bless you

I have a Matrix Treadmill t4. An error of "E1 and E3" occurs when it is started? Can you tell me the solution?

Have you tried to recalibrate the unit? Walking belt sticky at all? Smell anything? Does this use a red dc motor? If so, check for a burnt "choke" (looks like a small transformer near the controller bd). Elevation function at all? Can you get the walking belt to start?

1) If the the belt is moving during that time then stop I would suspect that speed sensor maybe defective. Replace the sensor
2) Now if their is no belt movement then either Motor Control board or Drive Motor is defective. Do Motor Test
I would check the drive motor first to eliminate an expensive component, unplug unit then remove motor shroud then unplug the connectors leading to the motor itself. If you have wire leads and a 18v drill battery connect the leads into motor then attach the other end to you battery and if the motor will rotate ( approx. 1mph) if that is accomplished then the only other component to be replaced would be the Motor Control Board. Call Matrix for parts @ 1-866-693-4863 option 3