The catalytic converter is a sensitive device with precious metals coating the inside. Without these metals, the redox reactions would not occur and the whole module would be useless. There are several substances and chemicals that inhibit the catalytic converter from doing its job.
Lead- most (if not all) vehicles run on unleaded gasoline, meaning all the lead has been taken out of the fuel. However, if lead is to somehow get inside the fuel and is burned, it leaves a residue that coats the catalytic metals and prevents them from contacting the exhaust fumes and performing the necessary redox reactions.
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