Where can i find a replacement magnet for my nordictrack a2105 model

Apparently the key is magnetic, so perhaps you had a magnetic event? Having said that, it shouldn't run unless switched on. Better to turn off at the mains when not in use.

https://www.google.com/search?q=nordictrack+a2550+pro+started+without+key

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The magnetic read switch is faulty . You have to replace the reed switch.If you wish to get some details; check the site linked here. Pull up older posts
http://electro-medical.blogspot.com/

this is a problem with Console Control with CPU, you must replace.
However you must sure if the signal coming out from the control motor to the upper board are o.k Otherwise time to replace the control board since, they send the signa from the inverter to CPU.

If the console only reads zeros or the controls do not function properly, try adjusting the reed switch. You will need a Phillips screwdriver to remove the motor hood and adjust the reed switch. Unplug the power cord, remove the motor hood and locate the reed switch, which is on the left side of the belt pulley. Rotate the pulley until a small magnet comes into view next to the reed switch. Loosen, but do not remove, the screw on the switch and move the switch with your hand until the gap between the switch and the magnet is 1/8 inch. Tighten the screw and replace the motor hood. The Controls Are Not Working on NordicTrack Treadmill LIVESTRONG COM

go to the next places

1) www.fitnessrepairparts.com

2) www.treadmilldoctor.com

God bless you

1) test the receptacle wall, plug a desklamp for to see if work if is working go
2( testing the power cord for open or defective.
3) reset the machine
4) test the fuse for blown and replace
5) test the c.breaker for tripped and reset again.
6) test the key, the key is small magnet, you can test with your magnet refrigerator with atraction or repulse, if doesnot replace the magnet from home depot.
God bless you

It sounds like the upper console has a loose connection ( check wire harness connections in the upper and lower board section ) or the upper console is failing call Icon Fitness @ 1-800-999-3756

I suggest checking your power plug then check or reset the circuit breaker on the treadmill next to the cord if that ok and the unit is still dead. Unplug unit the remove motor shroud and if you own a multi meter plug the unit in and start checking the AC all the way to the lower board and see where the AC is missing.
If the Voltage is good to the board then check DC Out put going the console wire harness should have 5v or 12v DC depending on model if missing then replace lower board if present, then check the continuity of wire harness if bad replace harness. If good upper console needs to be replaced

Is there a sheet of paper under the motor hood? It has the procedures for this calibration of the motor speed. Caution ! You should use a plastic screwdriver to make adjustment to the speed control on the motor controller. It is high voltage