implementation of in situ component tester

‘‘first off all I’am not execellent in english’’

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.

please i need help.

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…R

abouda:
but the problem is when i try to test them in the circuit there is a big difference.

please i need help.

Saying “there is a big difference” is about as useful as telling us how many children you have.

You need to tell us a lot more about what you have tried - and show us both the schematic for the test apparatus and the code you have used.

…R

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.