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Why don't cars come equipped with two engine coolant temperature sensors so the ECU can detect false ECTS readings-inaccurate readings but within system range.

I'm aware of cars that come with one sensor for the ECU and another for the dashboard. This has its own design problems but it's a story for another day.

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    Because an ECTS is $12, and easy to swap. But your idea that it only blows when 2 of 2 sensors fail is wrong. If 1 of 2 fails, the error is now "ECTS disagree" and blows twice as often. Whoops! Of course we could go to 3 sensors and disregard a minority report, but when sensor #2 fails the failed sensors are the majority. So now we're at 4 sensors and the ECTS disagree error pops on a 2/4 disagree? I think your real complaint is cars make no distinction between "fix today" and "fix by next oil change". Commented May 13 at 23:58

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I'd say the most likely reason is cost - yes a temperature sensor isn't particularly expensive (probably £15-20 retail so plucking a number out of the air.. call it £2 cost to the manufacturer) but multiply that by tens of millions of vehicles and it starts getting pretty big. And that's before you account for the need for the additional wiring on the loom, ensuring the ECU can handle the additional input, coding the software to handle two readings etc.

Secondly coolant temp is going to vary considerably at different points in the system - because that's what coolant does. It takes heat from the parts of the engine that produce it moves it to the radiator and dumps the excess to the environment before looping back to do it all again. You could place the sensors at say the hottest and coolest points of the loop and average them but that would mean that when one sensor fails you're going to get a seriously lopsided reading one way or the other and you've not gained much in a reliability sense, although a clever implementation could help spot coolant flow issues. Placing both at a similar point in the loop makes spotting failures easier for the system and gives you continued functionality in the event of one failure.

But how common are such failures and what's the consequences of a failure? They aren't super rare but they aren't failing week in-week out either (anecdotally in 20+ years of car ownership and god knows how many miles I've had a grand total of one ECT sensor failure), consequences can be bad (depending on the failure mode) if you don't spot the issue and have it it addressed promptly but they're rarely fatal to the vehicle and someone paying a basic modicum of attention to the gauges is going to spot failures before someone truly terrible happens so in those cases you aren't saving the owner any time or money because they need to take their car to the mechanic for the same thing - an ECT sensor replacement.

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Complexity and cost, for little benefit.

There's limited use in having 2 sensors instead of one - if one breaks then you have a disparity between the two, but the ECU doesn't know which one to trust so all it can do is flag up a warning and it still doesn't have a reliable source of temperature information. You'd need three sensors, so if one disagrees with the other two it would be obvious.

Now, multiply that by all the sensors; oil temperature, oil level, air flow and temperature, coolant temperature, AC refrigerant, upstream oxygen sensors, downstream oxygen sensors etc, etc and you have a lot of extra sensors and wires which add cost to the car, and make it harder to design and build. 3 sensors instead of 1 makes it 3 times more likely a sensor will fail, so you'll have worse, not better reliability.

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  • Three is even worse though. Yes, when 1 fails it's the minority and you ignore it, which is OP's goal. But when another one fails, now the failed sensors are the majority. Commented May 14 at 0:00
  • The idea is that a single failure would mean a check engine light @Harper-ReinstateMonica, so you wouldn't get that situation. Human nature and budgets would often mean you would get 2 failed sensors it is true.
    – GdD
    Commented May 14 at 7:33
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Because it doesn't make financial or technical sense. Adding just a dollar to the unit cost of a vehicle produced by someone like, say, Honda who makes 5+ million vehicles a year is a serious cost consideration.

Then the technical piece; what does spending those millions adding another sensor actually gain you other than an additional point of failure? Having one sensor die will theoretically throw a Check Engline Light (CEL) anyway.

In closing, Redundancy == cost & complexity and often it just doesn't make sense.

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They have engine coolant temperature fault detection logic instead.

Like all sensors that could affect emissions, OBD II regulations require fault detection. Broken wire (out of range) detection is easy. Cars have more complex fault detection logic, including detecting engines that don't warm up, or are abnormally hot after a sufficient cooldown period.

For example, see GM's detection logic on one specific family.

Engine Coolant Temperature Below Stat Regulating Temperature

P0128

This DTC detects if the ECT (EngineCoolant temperature) does not achieve the required target temperature after an allowed energy accumulation by the engine. This can be caused by an ECT sensor biased low or a cooling system that is not warming up correctly because of a stuck open thermostat or other fault.

This diagonstic models the net energy into and out of the cooling system during the warm-up process. The five energy terms are: heat from combustion (with AFM correction), heat from after-run, heat loss to environment, heat loss to cabin, and heat loss to DECO.

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