P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Anyway, if you have a modern multiport injection fuel system each injector opens specfically for its associated cylinder only, and the moment you shut the engine off the injector circuits are disabled and none open. There will be no fuel coming in.
If you have a very old carburated system there might be. In the early '70s, a few of the first emissions reduction efforts the manufacturers made was to increase engine operating temps (T-stats went from typical 165F to typical 195F) and to “lean out” the burn mixture. That resulted in cylinder temps so high that if there were any fuel in them it would self-combust when heated by compression. This process is called “dieseling”, because using compression generated heat to ignite the fuel is the way a diesel works. And since fuel was drawn in and metered by the air being pulled in every time a piston went down, for many engines that meant that when the key was turned off the engine would continue running until the fuel level in the float bowl dropped to a level insufficient to continue providing fuel. Engineers had to solve this tendency with “idle stop solenoids”. Thay totally shut off the air passage when the key was OFF via a circuit that got deenergized, and the engine choked.
To answer your question directly, the only way to possibly clean the oxygen sensors with raw fuel is to remove them and use a toothbrush. They still might not work but they’d be clean. Since it’s impossble to clean the catalyst, it probably wouldn’t matter anyway.