I have a healthstream treadmill model LS3T and it has just come up with a fault ER01 which the owners manual says is a communication error. Where should I start looking

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

Sounds like a fault in in the treadmills computer system the beep3 are error alert sound meaning somethings has gone wrong inside or the motor is reporting jamb fault code etc and its displaying with the lights upon the display board an error code a treadmill enginer will understand

Your model doesn't seem to be listed on the manufacturer's website. Contact them directly for additional support.
http://www.healthstream.com.tw/index.php?act=contact

go please to www.fitness.manualsonline.com or www. retrevo.com

and download for free.

God bless you

follow this steps and fix it. God bless you
Instructions

1. • 1 Unplug the treadmill. Locate the console key and place the safety or reset key into the treadmill console. Replace any parts that need replacing, or move on to the next step.
     
   • 2 Locate the reset switch on the underside of the treadmill console. Usually the button is large enough to trigger with a pencil eraser or Q-tip. Plug the treadmill plug back into the wall. Use the Q-tip to press the reset button. It may be necessary to hold the reset button down for a few seconds. "CL11" will appear in the display of the treadmill.
     
   • 3 Press the reset button one more time with the Q-tip. "CL21" should appear on the screen. Press the enter key. The display will change again and show "CL22." Keep pressing the enter key until the numbers cycle through to the number "CL26." Press enter one more time. The letter E should appear on the display screen.
     
   • 4 Press enter twice more. The letters "SS" should appear on the screen. When this happens, press the start/stop key. Making sure to keep well away from the tread, press enter to start the treadmill. The treadmill will start on its own. The treadmill will stop on its own after a few seconds. When it stops, press the power key two times. Remove the safety key and put in a safe place. Your treadmill is reset.
     

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

Hope this is what you need.http://www.healthstream.com.tw/upload/2011/01/20110128111522.pdf

If you need further help, I’m available over the phone at https://www.6ya.com/expert/matt_8dbc24bf722649ac

Issue might be that a magnet has fallen off. Here's an illustrated guide that should make fixing it easy: http://fixmytreadmill.blogspot.com/2010/10/how-to-fix-treadmill-e1-error.html

Good luck.

You will need to check the speed sensor for proper alignment and function. it should be located near the front roller or mounted on the motor. It should be within a credit cards width of the magnet or toothed wheel. If properly adjusted check with an ammeter to see if it is opening and closing by manually spinning the front roller or motor flywheel. If it isn't replace the sensor. If it is functioning make sure you are getting 5vdc to it across the wires. If you are not replace the upper control board(console). If you are getting the proper voltage then replace the lower control board(motor controller).