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What does the fail-safe mode mean?

(2005 Ford Freestyle)
in Cookeville, TN on June 17, 2011
I live in Tennessee and was driving in Dallas, Texas on a hot day and the fail-safe mode and engine light appeared on my dash. I pulled over and stopped the car and waited for 10 minutes. When I started it, everything was normal. I returned to TN and left the car with my wife in Dallas and she experienced the same thing; except the fail-safe mode remained on the dash for two days.
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Popular Answer
If the check engine light is on the computer in your car sees a fault which needs to be investigated. Even if the light goes out the fault will probably be still stored in the computer of your car. Fail safe mode is the computer in your car trying to minimize or prevent serious damage by altering transmission shift, engine RPM or engine load conditions.
on June 17, 2011
The strategy behind the "Fail Safe" or "Limp Home" mode of operation is to ensure that the engine is still operational in case of a worst case scenario: Your stuck in the middle of a desert, cellphone is dead, rattle snake bit you and you seriously need to get to the next town. Walking isn't gonna help...

But even more so to PROTECT the engine itself.

If a critical sensor, like an air flow meter, is faulty, then the engine is at risk of running too lean. When an engine runs too lean, it means that there is more oxygen upon combustion than there is fuel to burn. That extra oxygen is now placed in a high pressure, high temperature environment and it is gonna want to react with something. That's when the Oxygen atom says to his buddy: "Hey, lets walk over to those Aluminum atoms at the bar...?". Thus when the engine runs too lean, then you could potentially burn a hole through your pistons or weld the pistons to the sleeve of your engine-block.

(You might vaguely recall from primary school science experiments that even iron burns in pure oxygen, what happens here is not too different)

For this reason the computer compensates by making sure that the engine is ALWAYS running far too rich. Hence it wont damage your car whilst driving in this mode because it is intended to protect your engine. Cant say the same for your fuel budget though...

Basically, the computer has detected a problem so perilous to the engine that is only runs the engine to the extent absolutely needed to make it from point A to point B.

Bottomline.... get that thing checked out.
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