hello my project is to make an in situ component tester with arduino mega . it’s easy to test components
off circuit (alone) but the problem is when i try to test them in the circuit there is a big difference.
I'd have thought it was clear that testing components which are soldered together on a circuit board could yield different results to testing them individually. For example, two 10k resistors in parallel (admittedly unlikely) would measure 5k on the board and 10k separately.
But that lies in the nature of your testing exercise and choice of test instruments. It is not specifically an arduino problem. That is, unless I have missed something.