Check Engine Light - and Gasoline (car engine, spark plug, fuel, MPG)

[Samana]

Is it true that when your "check engine" light is on - it's uses up a lot more gas? Also, can a 'check engine' light have to do with the transmission going bad?

[DanBev]

My check engine light was caused by ethanol plugging up my catylitic converter,back pressure,oxygen sensors.Not nice that owners manual says not to use ethanol.
My gas mileage is 3-4 mpg less on ethanol.

[MechanicalDoll]

Originally Posted by Samana

Is it true that when your "check engine" light is on - it's uses up a lot more gas? Also, can a 'check engine' light have to do with the transmission going bad?

Your car might use more fuel, but it depends on the problem which set off the check engine light off. So just because the light might be on, doesn't always mean you'll be burning more fuel.

And yes, transmission problem could cause your computer to generate a trouble code and set off CEL.

[MechanicalDoll]

Originally Posted by DanBev

My check engine light was caused by ethanol plugging up my catylitic converter,back pressure,oxygen sensors.Not nice that owners manual says not to use ethanol.
My gas mileage is 3-4 mpg less on ethanol.

Actually your manual kinda does tell you not to use it. It says to use unleaded regular 87 (depending on the car)! Which means you should only use that...its common sense. Just because it doesn't say "do NOT use diesel" are you gonna go out and put some diesel into your tank? E85 is ETHANOL (85%), which makes it different kind of fuel than regular gasoline.

On the other hand if you CAN use E87, the manual WILL tell you. It will indicate that you have Flex Fuel system can be run on E85.

And it's a known fact that E85 provides lower mpg than gasoline.

[yellow4yield]

Modern car engines have some very complex computer controlled systems in them. And the engine computers have the ability to "self-test" various systems.

And have "back-up" systems for some things should one of these systems fail.

In certain situations, the self test might detect a failure with a system - like an input to spark plug timing. Then set the check engine light and place a diagnostic trouble code in the computer memory. And then set the engine timing to a default "Limp Home Mode" which will allow you to still drive the car.

In that case you would not have the optimum timing or engine performance and it would use more gas, but it would run.

However the check engine light could be on for all sorts of reasons. Maybe it is...
DTC P0565 Cruise Control On Signal Malfunction

Or any number of other reasons. Many of which would have nothing to do with gas mileage.

Here is a list of trouble codes...
OBD-II Check Engine Light Trouble Codes

FYI - As to which codes would be associated with a "limp home mode", good luck finding that information! This is sometimes mentioned in a Factory Service Manual, but will be buried in text for a specific system or troubleshooting section.

And this has to do with the programming in the engine computer. That they keep top secret or feel that no one needs to know this information (they intentionally "dumb it down"*).

*If they want to get intelligent people to be car mechanics, they had better start making this technical information available! (Like the rest of the electronics industry does...)