98 honda civic fluctuating idle

There’s no fundamental need for the O2 sensors. The car is designed to run well with no O2 sensor input, b/c it has to do that until the O2 sensor heats up enough that the signal becomes valid. Nobody would accept a car that runs poorly until the O2 sensors heat up.

It’s just that the fuel to air mixture won’t be optimized perfectly without the O2 sensor reading, which could cause carbon build up (if rich) or potential overheating (if lean). I expect OP already realizes the federal emissions implications of such a modification.

Not quite. The temp sensor tells the ECU that the engine is still cold and the ECU actually ignores the oxygen sensor input. Once the car warms up, and the ECU sees that from the temp sensor signal, the upstream oxygen sensor becomes a variable in the algorithm and the ECU also begins comparing its signal with the downstream sensor.

This is necessary because a cold engine needs to run rich to operate. The extra fuel in he mix compensates for the reduced power in the combustion caused by the cool cylinders. The engine can’t use the full energy available in the gasoline if the combustion process starts out cold.

Carbon deposits are a problem for cars in cold climates that never go farther than the corner store. Always have been, more so with carbs (which run richer than EFI anyway), but also with EFI.