You can read our guide to rejecting a vehicle here - www.honestjohn.co.uk/faq/consumer-rights/ but in brief, under the Consumer Rights Act 2015, consumers have a 30-day "short-term right to reject" faulty goods for a full refund. This applies if goods are not of satisfactory quality, fit for purpose, or as described. The 30-day period begins upon delivery or transfer of ownership. We would however suggest giving the supplying dealer at least one opportunity to rectify the fault. In the event that they attempt but fail to solve the issue then you can reject the car, but we would recommend contacting the dealer in the first instance and reporting the issue.