New cars have H2OSs to warm the O2Ss. Is this, and any other new tech in new cars, enough to remove all the negative effects of a cold start? How does a cold engine behave differently now (2019) than it might have done 30 years ago? I am quite new to engines and car maintenance but I understand that the H2OSs remove a lot of time in open loop and thereby uses the sensors more quickly to manage fuel and air mixtures. How else is the fuel efficiency or potential damage affected by a cold start?

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It's true that heated O2 sensors shorten the time the engine is in open loop. So the whole "wasting gas thing" while you wait for the engine to warm up isn't as big an issue. But there are many other reasons to not sit and warm up your engine. The most important one is that warming your engine will warm the coolant, but it doesn't warm the oil as quickly as if you were to just drive it. And oil is far more important than the coolant. Cold starts cause the most engine wear. But You get oil pressure within a few seconds of starting, even on the coldest days. Once the oil psi light goes out, you have enough psi to prevent metal to metal contact and wear. What you don't have is a maximum oil FLOW. It's a myth that you get better oil flow by sitting and warming the engine. Warming the engine doesn't do squat for the oil sitting in the oil pan. It's cold as hell. Whatever heat the oil picks up by circulating through the engine at idle is immediately lost as soon as that oil returns to the ice cold oil pan. So start the engine, buckle, turn on the defroster, find your radio station and then DRIVE. But don't put the pedal to the metal for a few blocks until you get better oil flow. There are a few exceptions to this rule and they pertain to some European engines that utilize spray lubrication in the crankcase. Those car makers do want you to wait and warm up the engine. See this post for the full pros and cons of warming an engine in cold weather.

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    If it's really cold out you may need to let it warm up a minute or 2 for the transmission to function... Oct 29, 2019 at 17:05
  • How cold would really cold be for the transmission to seize up? -20? -30?
    – atreeon
    Oct 30, 2019 at 8:27
  • Who said anything about seizing up? Most late model cars have transmission oil temperature sensors. By knowing the trans oil temp, the computer can adjust pressures and shift solenoid timing. Idling contributes very little heat to a transmission.
    – user9181
    Nov 1, 2019 at 16:32

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