If a fault manifests itself then you should immediately write to the seller and inform them of the fault and tell them that you are asserting your 'short-term right to reject' under the Consumer Rights Act. Even if this is not what you eventually want to do, at least by giving written notice you are reserving your position so you should always do it.
Best to write a recorded delivery letter but also send an email as well.
With sellers such as second-hand car merchants, don't expect things to be straightforward. They will balk and will make excuses and they may offer a repair instead. Your best interests will normally be to simply get rid of the item – especially if it is a motorcar.
Even reputable sellers such as some of the large computer sellers do not appear to be aware of your 2015 rights and will refuse to respect them without a lot of pressure.
Act quickly. Make sure within the 30 days you have served notice in writing and then seek our advice.
Follow this link to see section 20 of the Consumer Rights Act which creates the 'short-term right to reject'
