E14 error in a Brother embroidery machine typically indicates a malfunction in the upper thread tension. Here are the possible causes and solutions for the E14 error:

1. Upper thread tension is too loose: Check the upper thread tension and adjust it as necessary.
2. Upper thread is tangled or not threaded correctly: Check the upper thread for any tangles or knots and re-thread the machine.
3. Upper thread is too thick or too thin for the needle and fabric being used: Use the correct thread weight and needle size for the fabric being embroidered.
4. Upper thread guide is not threaded correctly: Check that the upper thread is properly guided through all thread guides.
5. Upper thread guide is bent or damaged: Check the upper thread guide for any damage and replace it if necessary.
6. Upper thread sensor malfunction: If none of the above solutions work, the upper thread sensor may be malfunctioning and may need to be replaced.

If the error persists after trying the above solutions, it’s best to contact a Brother authorized service center for further assistance.