Thank you. Symptons are it won`t kick in initially when peddling, eventually it will but will cut in and out. If you stop it either won`t work or surge and stop working again. I believe it has a sensor.
The competency of the bike shop is unknown as we haven`t used them previously but they do have very good reviews.
They're not the symptoms of a defective motor.
The most common cause for that is the motor not in ALL THE WAY to the MARKED LINE. In tight is not enough.
After that, it's the pedal sensor knocked out of line. See here:
Hi there, First ever e-bike, had it a couple weeks. The motor constantly feels/sounds like it's constantly restarting every pedal rotation. It's not smooth and feels like if you were to move your foot constantly up and down on your car accelerator. Is this normal?
www.pedelecs.co.uk
Third common cause is defective brake switches. Disconnect the brake electrical connectors to see if it goes away. You can run your bike without the brakes connected to the electrical system for testing. Some people like to ride with a finger or two on the brake. That doesn't work on an electric bike with brake switches because it cuts the motor.
You need to ask the shop how they came to the conclusion that the motor needs replacing. On the surface, it sounds like they've no idea about how to fix ebikes, but maybe they tested something somehow.