No power model # G-6448N the power cut out whilst treadmill was being used, now there's no power whatsoever. Its not the deadman magnet or anything obvious, i think the power board or EMI filter or something else has come a miss

You can try using a strong refrigerator magnet. It's not coded or anything.
Otherwise, you can go to http://www.treadmilldoctor.com/treadmill-parts-model-select and pick your brand and model and order the part. Probably under "accessories" somewhere.
The treadmill should have a label with the model and part number.

Adjust the REED swicht , see diagram, clean it, and tight the plug very well.

2) clean the ribbon cables, for keypads, clean it.
3) tight the connector from display board vs lower board too
see the block diagram
After you fix it. RUN the software calibration for diagnostic, troubleshooting. too
God bless you

How to Troubleshoot a Nordic Track C2500 X Jolie Johnson

Based in Austin, Texas, Jolie Johnson has been in the fitness industry for over 12 years and has been writing fitness-related articles since 2008 for various websites. She received her Bachelor of Arts in English and philosophy from the University of Illinois.
By Jolie Johnson, eHow Contributor

updated April 15, 2011

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The NordicTrack is a treadmill designed for in-home use. It offers a speed range of 0 to 10 mph and an incline range of 0 to 10 percent. It folds up vertically to save space when you aren't working out and can be rolled to a different room. The console has 16 preset workout programs and features iFit interactive technology. If you have problems with the console or walking belt, you can often fix them yourself with some basic troubleshooting steps
Instructions

1. Power Turns Off When You're Using the Treadmill
   • 1 Check the reset switch on the bottom of the treadmill near the power cord. If it has tripped, the top of the switch will protrude. Wait 5 minutes then press the switch.
     
   • 2 Check that the power cord is plugged in completely. If it is plugged in, unplug it, wait 5 minutes then plug it back in.
     
   • 3 Remove the magnetic key from the console then reinsert it.
     
   The Console Won't Turn Off
   • 1 Insert the magnetic key into the console. Your C2500 is stuck in demo mode if the console won't turn off after you remove the magnetic key from the console.
     
   • 2 Hold the "Stop" button down for a few seconds.
     
   • 3 Press the "Speed" down (-) button. Remove the magnetic key.
     
   The Incline Does Not Change Correctly
   • 1 Press and hold the "Stop" and the "Speed" up (+) button. While holding the buttons, insert the key into the console, then release the buttons.
     
   • 2 Press either the "Incline" up or down arrow button. Wait while the incline raises and lowers. This recalibrates the incline system.
     
   • 3 Remove the key from the console.
     
   Walking Belt Slows
   • 1 Unplug your C2500 and remove the key from the console.
     
   • 2 Turn both rear adjustment bolts, located on the back of the walking deck, to the left, a 1/4 turn.
     
   • 3 Lift the edge of the walking belt in the middle of the deck. You should be able to lift it 2 to 3 inches off the walking platform. If the belt is still too tight, loosen the bolts another 1/4 turn each.
     
   • 4 Plug the treadmill in and reinsert the console key. Walk on the belt at a slow speed to see if it still slows. Repeat Step 2 if necessary.
     

If the console takes batteries, replace them. If this does not work or your console does not require batteries, check the reed switch or optic sensor. Reed Switch: Unplug the treadmill and remove the motor hood cover. The reed switch is mounted on the frame next to the front roller pulley. There is a magnet glued into the front roller pulley. When adjusting the reed switch, make sure it is pointing directly at the magnet. Move the tip of the reed switch within 1/8" of the magnet. Also, check to see that the wire is not damaged, and is connected to the console
Optical Sensor: Unplug the treadmill and remove the motor hood cover. The optic disk and sensor are located either at the end of the motor or the right hand side of the front roller. The optic disk spins between two sensors on the optic switch. The disk must spin freely without touching either sensor. The sensors can be moved by loosening the adjustment bolt holding the switch to its bracket. The switch can then be slid. Center the disk.

The amps on the unit have probably died. Did they replace it for you? I'm becoming alarmed at the number of units just dying for an $800 piece of equipment.

Watch this video and Step:
http://www.youtube.com/watch?v=E-q-uS4t6so

Ensure that the treadmill is plugged into a grounded outlet using the correct surge suppressor. Use a UL 1449 suppressor listed as a transient voltage surge suppressor (TVSS). It must be rated for 15 amps and 120 volts AC. Secure the services of an electrician if the outlet is not grounded, or move the Proform treadmill near a grounded outlet. Do not use an extension cord.

• 2

Insert the safety key fully into the display. Treadmills with keys are designed not to turn on without the safety key in place.

• 3

Find the circuit breaker on the front of the frame and make sure it is not tripped. The circuit breaker is a vertical switch on the 540S and a horizontal switch on the 540 Interactive Trainer. Press it to the "Reset" position, which will be labeled.
2. Reed Switch

o 1

Unplug the treadmill and remove the safety key.

o 2

Move the upright base of the treadmill down with the help of another person and lift the uprights so that you can get to the underside of the console. Use a Phillips screwdriver with at least a 5-inch shaft to remove the screws. There are five hood screws on the 540 Interactive trainer and four screws on the 540S.

o 3

Remove the motor hood to get to the small reed switch by pivoting the hood off the frame. Find the reed switch and magnet, which are on the left side of the pulley for both models.

o 4

Turn the pulley with your hand until the reed switch aligns with the magnet. There should be 1/8-inch gap between the two. Fix the distance so that it is 1/8 inch by loosening the reed switch screw and sliding it to the correct distance. Tighten the screw.

o 5
Reverse the directions to put the hood back on and the treadmill upright in the original position

Chech: Optical Sensor if have it.

Make sure the magnetic safety key ('safety pull' or 'emergency stop magnet') is in place. SAFE OUT indicates that the safety key is not in position or is otherwise not working properly.

For more information consult the following manuals:

http://download.dfxi.com/supportdocs/OM/Bowflex/BFX_TM3-5-7_OM_RevD_web.pdf

http://www.pdfstock.com/bowflex-3-5-7-series-treadmills-assembly-manual-pdf.html

I would think that it can't hurt anything to try another magnet but if you end up needing to buy another one here is one website that sells them: Sportsart TR20F safety key

Is there a key that has to be inserted in the control box? this would act as a deadman switch so that when it gets pulled out the machine stops. without it the machine won't start.