The check engine light is a commonly misunderstood piece of equipment on a car. The light comes on and the driver freaks out, thinking the vehicle is about to blow up. If you understand what it's supposed to do, you also can anticipate what kinds of things unexpectedly cause it to come on and what you can do about it.
The check engine light is more tied to emissions than anything else. That's not to say other things might not cause it to come on, but it most common activates when the computer senses something out of balance with the emissions. This means its operation is linked strongly to the oxygen sensor.
The oxygen sensor detects the level of oxygen in the post-combustion emissions, and is looking for values within a certain range which tell the computer that everything is working properly - the engine is burning the fuel in the right way and the emissions are in the proper balance.
When the oxygen sensor detects something amiss, based on what it "sees" in the emissions, it can tell the computer to adjust certain operational things (like timing) to see if it can correct the problem. If the problem isn't correct, the check engine light comes on, which is the computer's way of telling you "I can't fix this by myself". You take it to a mechanic, they plug a code reader into the car's computer, and the computer tells the machine what things it's seeing. This gives the mechanic some strong clues on what they need to look for to correct any particular problem.
Check Engine Light On? Check Your Fuel
Believe it or not, the ethanol in your gasoline can be a trigger for the check engine light. Or more correctly, if the computer is expecting to see one thing and it keeps seeing another, it will cause the computer to flip on the warning light in response.
Step back for a moment and recall that, for non-flex fuel vehicles (i.e. probably your car), they're designed to run on ethanol percentages in fuel of 15% or less. They're not designed to run on, say, 85% ethanol fuel (E85) - and by this, we mean the timing and injection in the engine are set to function properly on lower concentrations of ethanol. Why does this matter, that your car isn't set up this way? If you put E85 in a vehicle where the computer is expecting to see 15% or less, the level of oxygen in the emission that the sensor reads is going to be dramatically different. This confuses the computer, which is not expecting to see levels of oxygen as high as that. The computer thinks it's not supplying enough fuel and starts injecting too much fuel for what the engine really needs, while also playing with the timing by changing how often and when the spark plug fires. In most normal cars (i.e. non flex-fuel), it wouldn't even run in this kind of situation. (Continued Below)
But take this same explanation and take E85 out of the equation. You're at the pump where the sticker says "contains up to 10% ethanol". But maybe someone at the fuel supplier mixed the ethanol-gas wrong, and it really contains 21% ethanol or 23% or 18%, instead of the ten percent the car is expecting. And, worse yet, what if the gas station management is "unscrupulous" and they decide to substitute methanol instead of ethanol? Why would they do that? Methanol is cheaper and they'll save money. But it contains more oxygen by weight than even ethanol does.
And so, in both of these highly-plausible scenarios, the computer sees more oxygen in the exhaust than it thinks it should. But it's not so far out of whack that the engine can't run. It's just not going to run nearly as well on 21% ethanol or 17% methanol instead of ethanol. So the computer starts playing with the timing and the fuel injection and you end up using more fuel than you should (lower mileage). And if it can't correct the problem that way, it will turn your check engine light on.
You may be interested in these other posts:
- Wait! Should you buy a used car? Check out these signs you shouldn't.
- Beware of certain types of ethanol gas treatment
- Falling Gas Prices Impacting Ethanol For Consumers
This post was published on October 30, 2013 and was updated on February 20, 2015.