p0420 after full exahust install
 

I installed bbk longtube headers with bbk mid pipes with high flow cats and dynomax cat back put about 1k miles on the car. At about 500 miles in the rain I got code p0420 I cleared it after checking to make sure my wire connections were good. At 600 miles again in the rain the code returned I again guessed water in the wire connections, replaced wire harness and this time at 1000 miles in dry conditions code returned but I can clear it. Shop suggested shimming the o2 sensor........where do I get the shims and how do I do this?