The fault itself:
Do not try to fix anything, or have anybody try to fix it, interfering with it in any way may affect your rights.
Do not clear any codes or delete anything with a scanner
The moment you say to the finance company, dealer or whoever “
My m8 big-john is a proper good mechanic like, and he had a look and tried XYZ, and pulled out the ABC and tightened up this and that” you’re basically attacking your own case with a sledgehammer.
In my case the finance company sent an independent professional vehicle inspector to get a diagnosis. This guy couldn’t disassemble or touch it, he simply looked at the symptoms and gave an unbiased opinion to the finance company, but it’s his opinion which is one of the most important parts of the whole process, as that will guide the finance company with regard to making a decision.
Generally, the fault has to be “significant” in the sense that it directly affects the general operation of the car. There are debates over what constitutes “significant” however it wouldn’t be a flat tyre or a faulty electric-window motor.
The vehicle has to be “fit for purpose” “as described” and “of satisfactory quality”. Even if you buy a 14 year old X5 for £5k, (

) the same legislation protects the buyer, as if it was a brand new X5 bought for £80k. You won’t be in a situation where your rights don’t apply, “
because you bought a knackered car and should have known better” the law is the law and the dealer is either in breach of contract, or they’re not.
If you bought this via cash or bank loan, directly from the dealer - you’ll still need to prove that the fault exists, in that case - you’ll need to pay for your own vehicle inspector to provide a report. This can get complicated because when you return the vehicle - the dealer can contradict that with his own inspector, if they decide not to refund - you’ll need to take legal advice from a solicitor and potentially go down the small claims route.
If you paid for some or all of the car via a credit card, speak to the credit card provider immediatley, as they have a standard process for dealing with this, and they hold all the power.
Note: If you attempt to, or actually fix the vehicle - you lose any right to reject unless it fails again within the 30 days since purchase. So it goes without saying: If it breaks and you want to reject it, don’t **** around with it, under any circumstances whatsoever.*
Note2: You’re always going to have an easier time if the inspector deems the fault to have been preexisting or developing at the point of purchase. But sometimes things just go bang due to the nature of these machines, in either case you should be fine - as under the CRA a vehicle has to be fit for purpose. But seek some professional legal advice if in doubt there as it is a bit of a grey area.*
Note3: Don't be intimidated by legal threats or letters, the situation is simple: If you bought a car from a motor trader and it broke in a week or two or whatever, they're liable to either fix it or refund all of your money, they essentially have no leg to stand on.
Note4: With regard to inspection and diagnosis, don't use a regular mechanic - use a professional vehicle inspector. These guys write officlal reports for dealers, finance companies and insurance companies, they'll inspect/scan the vehicle without altering it in any way. If this ever goes to a small claims court, using a professional inspector will always hold more weight than the opinion of a random garage.