My 2010 Ssyangyong Actyon sports ute often suffers a lot of power loss while driving, which can be dangerous if it happens when you're overtaking. A dealer replaced the brake light switch, which seemed to improve it, but didn't fix it. The dealer checked it, but could not find a problem. Shouldn't they be able to put them on a computer that diagnoses the fault? What can I do to make them honour their warranty, or can they just stall till it expires?
They can, and I am confident that they have had it on the computer, but they can only identify problems that exist when the car is put on the machine. Yours is intermittent, so might not be evident at the time the car is tested. It's most likely a faulty engine sensor that is only occasionally playing up.