I have a Neff microwave(model.C54L70N0GB) Frequently it will cut out before the dialed in time, beep and give an E305 error. Any suggestions?

Time to replace the door switches.

Unfortunately that error is not listed anywhere.

As it is also a microwave oven, it need to be fixed professionally.

https://www.google.com/search?q=NEFF+combination+oven+microwave+error+E1+15

This is what Neff say. I am pleased they just asked me for my Feedback, so I can tell them what I really think of their website help. :>D

https://www.neff-home.com/uk/service/support/ovens/error-codes

..

Constant beep sounds like a bad microprocessor on the main time board. May have to replace the board.

You may need to replace the keypad. I found there was moisture causing the problem when we boiled water on the right side of our stove which is under the keypad. I let the microwave dry out for several days and now only use the left side of stove when we boil water and have not had the problem.

I notice you did not list the make and model, but in general they beep 3 to 4 times or more, it is not uncommon for some to give more. If you have your owners manual there might be a option offered, or contact the manufacture and talk to customer support and the technical department for a possible solution.

Possibly the magnetron is not producing power and as the cavity temperature is not rising, it cuts out.
Is there an error code in the display?

This is happening due to either the magnetron, or the Power Diode, or both, is breaking down, under load, then the unit detects the fault condition, & then, for safety reasons, to avoid a fire or explosion, shuts down the system. You need to take it in to be looked at please obtain a "Quote" before proceeding with any work. This way you can decide to repair or replace.

check on rating plate if fd number starts with a 7 or 8 eg fd 7908 then appliance will need mag leads recrimping they oxidise over time and break connection giving syptoms that you describe, its an easy fix but you need to remove outer casing.

Take it back to where you bought it from and get a new one!

Finally worked out how to do this today! I don't know what model you've got (I don't know what one I've got - it's built in and we've lost the manual) but try pressing the START button for six seconds. This should change the default from 30 beeps to 3 beeps. Bliss.