The situation you describe sounds a lot like what I was experiencing – having the O2 sensor replaced fixed my problem. My car would run normal while the engine was cold; I assume I was seeing the problem when my car transitioned out of open loop mode because when the engine stubmled I noticed the temperature quickly rising to normal operating termperature. Shortly after the temperature was up to normal operating temperature, the car would run fine. The O2 sensor data isn’t used in open loop mode (engine is cold or under high load), but is used during closed loop mode to adjust the fuel trim (ratio of fuel and oxygen supplied to the engine) – causing the engine to run more lean or rich as needed. From what my mechanic told me, I assume that although my O2 sensor was not bad enough to be picked up by the engine diagnostics or the scan tools I used, there must have been some problem with the response or voltage from the O2 sensor and therefore the ECU would cause the stumbling I observed until some type of normal state was achieved. That’s my guess about the problem I was seeing – if my mechanic hadn’t been able to find the cause and I had to choose between changing the O2 sensor or the cat, I would have tried the O2 sensor first since it is easier and much cheaper to replace than the catalytic converter.

I have a 1996 Honda Accord with 265k miles on it.

I had the speed sensor go out and I replaced that and the speedometer still works only sometimes.

If I start the engine and drive without letting it warm up, it starts to stall, like there is no gas getting to it, but it never really stalls, just jumps, stops, goes, not fun at all.

That was this morning after warming it up 10 minutes in 40s temp. But five more minutes and it ran normal. Typically, in the morning, if I run it 15 min to warm up, there’s none of that hesitation.

Codes show speed sensor still and and the catalytic converter (0420). Took to a great Honda tech and says he noticed nothing (cause engine was warm) and says couldn’t be 02 sensor since that wouldn’t do anything to the driving capability.

I don’t know what to do if I’m not getting an 02 sensor code. By the way, the mechanic said the catalytic converter wouldn’t cause that problem either.