Can exhaust doughnut gaskets be reused if they ard still in good shape or are they a one time use crush part? Is it possible that if they do leak, could that affect O2 sensor readings at idle due to reversion? I am running some old TES headers which have the O2 sensors in the driver side collector upstream but near a doughnut flange. I have a weird issue where my fuel mixture at idle is good in park but lean in drive and wondering if it could be false reading due to exhaust leaks? I already replaced the exhaust gaskets between the heads and headers so the seals there are good.