I am not a lawyer; I am not your lawyer.
From the Sale of Goods Act 1979:
(2)Where the seller sells goods in the course of a business, there is an implied term that the goods supplied under the contract are of satisfactory quality.
(2A)For the purposes of this Act, goods are of satisfactory quality if they meet the standard that a reasonable person would regard as satisfactory, taking account of any description of the goods, the price (if relevant) and all the other relevant circumstances.
(2B)For the purposes of this Act, the quality of goods includes their state and condition and the following (among others) are in appropriate cases aspects of the quality of goods—
(a)fitness for all the purposes for which goods of the kind in question are commonly supplied,
(b)appearance and finish,
(c)freedom from minor defects,
(2C)The term implied by subsection (2) above does not extend to any matter making the quality of goods unsatisfactory—
(a)which is specifically drawn to the buyer’s attention before the contract is made,
(b)where the buyer examines the goods before the contract is made, which that examination ought to reveal, or
(c)in the case of a contract for sale by sample, which would have been apparent on a reasonable examination of the sample.
Based on my interpretation of this, it would depend on the information about the car you were provided at the time you bought it.
- If the information you were given was such that a reasonable person would expect the catalytic converter to be faulty, then the Sale of Goods Act is unlikely to serve your interests, in this case.
- If the representations were that the car was fit for purpose, however, and a reasonable person would have expected it to run without fault for some reasonable amount of time, then you may be able to bring an action against the dealership.
Note: The fact that a reasonable person would come to the conclusion that the fault was pre-existing is irrelevant if it was disclosed ahead of time. Additionally, the fact that it was a used car may change what would be considered reasonable in this case.
In either case, I suggest you seek legal advice and/or a second opinion from another mechanic on your car. You may be able to find out more about whether this is, in fact, something that would be expected.
Also, keep all documentation you have, including any advertisements for the sale of the car.