Shalom, I use a plastic sink liner on top of my kitchen sink for Pessach. I first kosherize my sink and then put the liner on top of it. This year, a pipe got clogged and residues from the inside the pipe reached the surface of the sink, but did not reach the plastic liner. I wonder if such residues are considered chametz. Is it a problem to wash dishes now using warm water from the tap? What about cold water? Could this water somehow "bring" the possible chametz from the inside of the sink to my plastic liner?

Shalom. Residue from the sink may still be considered chametz, although in all probability, while cleaning for Pesach you inevitably poured a lot of soap and cleaning agents which rendered the residue ‘not suitable for a dog’s consumption”. In any event, if you’re using hot, not boiling water, it’s no problem anyway, because absorbed chametz can only be “expelled” and transferred when the water is so hot, that your hand instinctively jumps back. In addition, the fact that you use a liner, adds additional stages between the residue (even if edible), until it “enters” the Pesach dishes. In short, no problem. Hope you solve the clog problem anyway by next Pesach, and that you’ll be in Jerusalem anyway! With Love of Israel, Rav Ari Shvat