The ASA might compel it to. A dealer is responsible for any fault that could have been present or developing on the date of sale for six months from the date of sale: www.honestjohn.co.uk/faq/consumer-rights/. A warranty covers any new problem that develops after the date of sale. It cannot cover faults that were present on the date of sale because that would be fraud, in the same way that you cannot take out fire insurance on a building that is already burning. However, I do not recommend RAC Approved used cars for a different reason: the number of complaints I receive about the RAC refusing to repair cars it has warranted usually quoting “wear and tear". If a warranty does not cover components such as alternators because they have “worn out”, then the warranty is of no benefit when buying a car.