Ford Fiesta st 2014

[Ryan milne]

My fiesta st won’t start, put a new battery in worked fine for a few days now it’s dead again, checked the usual things nothing wrong, it’s a push button start so I was wondering if the key battery was knackered that could cause it to not start ? 
 

[mjt]

If the key battery was failing you should get a warning on the display and you should still be able to start by inserting the fob into the space normally occupied by the ignition lock (I'm assuming the Fiesta will be the same as the Focus). What "usual things" have you checked? Have you tested the battery voltage with and without the engine running?