Can I be found liable if I part-exchange a car that I suspect has faults?

I'm part-exchanging my car for a new one (to a garage). He turned the car on and back off and said his checks were done, but I think there's an issue with the EGR. However, I'm not a mechanic. Do I need...

What's the best time to part-exchange a car?

We want to get rid of our Skoda Karoq before the manufacturer's warranty runs out on 28 February next year. When do you think the best time would be to p/x between now and then?

Do I need to disclose an engine replacement when part exchanging a car?

I have recently had a new engine fitted to my car under the manufacturer warranty at just 13000 miles. When i come to part exchange it am i obliged to tell the dealer about it?

Honda CR-V (2007 - 2012)

Roomy and versatile. Very comfortable, pleasant and relaxing to drive. not at all cumbersome. 2.2 i-CTDI diesel is excellent.