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I did a body swap on my iphone se . Everything works perfect except i now have very poor cellular reception.one bar where it should be five bars. I disconnected the cell antenna and it made no difference . I guess this proves the antenna is not working. Assuming it is something i have done to cause the problem . Thanks for any ideas on this problem

Most likely the body. If the replacement body does not have paint sanded off on screw holes and electrical contact surfaces, it will not have good reception. Also make sure you put back every washer or contact clip where they should be, any one of them could affect reception.