How The Check Engine Light Works

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Have you ever had an experience like this? You drive through the one of those automatic car washes. When you get to the end, where the dryer is blowing, your check engine light started flashing!

You fear the worst, but within a block or two, the light stopped flashing, but stayed on. By the next day, the light was off.

You wonder; “What was going on?” Well, it’s actually a good lesson in how the Check Engine light works.

Your air intake system has a sensor that measures how much air is coming through it. When you went under the high-speed dryer, all that air was blasting past the sensor. Your engine computer was saying, there shouldn’t be that much air when the engine is just idling. Something’s wrong. Whatever’s wrong could cause some serious engine damage.

Warning, warning! It flashes the check engine light, to alert you to take immediate action.

It stopped flashing because once you were out from under the dryer, the airflow returned to normal. Now the engine control computer says the danger is past, but I’m still concerned, I’ll keep this light on for now.

Then the Check Engine Light goes off in a day or two.

The condition never did recur, so the computer says whatever it was, it’s gone now. The danger is past, I’ll turn that light off.

Now a flashing check engine light is serious. You need to get it into the shop as soon as possible. But if it stops flashing, so you have time to see if the problem will clear itself or if you need to get it checked. How does the computer know when to clear itself?

Think of it this way. The engine control computer is the brain that can make adjustments to manage the engine. Things like alter the air to fuel mix, spark advance, and so on. The computer relies on a series of sensors to get the information it needs to make decisions on what to do.

The computer knows what readings are in a normal range for various conditions. Get out of range, and it logs a trouble code and lights up the check engine warning.

The computer will then try to make adjustments if it can. If the computer can’t compensate for the problem, the check engine light stays on.

The computer logs a trouble code. Some people think the code will tell the technician exactly what’s wrong?

Actually, the code will tell the technician what sensor reading is out of parameters. It can’t really tell you why, because there could be any number of causes.

Let’s say you’re feeling hot. You get your heat sensor out – a thermometer – put it under our tongue and in a minute or two you learn that you have a fever of 104 degrees.

You know your symptom – a fever – but you don’t know what’s causing it. Is it the flu, a sinus infection or appendicitis?

You need more information than just that one sensor reading. But it does give you a place to start and narrows down the possible problems.

There are reports on the internet telling you that you can just go down to an auto parts store and get them to read your trouble code or buy a cheap scan tool to do it yourself.

There are two problems with that. First, the computer stores some trouble codes in short term memory, and some in permanent memory. Each manufacturer’s computer stores generic trouble codes, but they also store codes that are specific to their brand.

A cheap, generic scan tool, like you can buy or that the auto parts store uses, doesn’t have the ability to retrieve long-term storage or manufacturer specific codes. Your service center has spent a lot of money on high-end scan tools and software to do a deep retrieval of information from your engine control computer.

The second problem is that once you’ve got the information, do you know what to do with it? For example, a very common trouble code comes up when the reading on the oxygen sensor is out of whack.

So the common solution is for the auto parts store to sell you a new oxygen sensor, which are not cheap, and send you off on your way. Now your oxygen sensor may indeed have been bad and needed replacing. But the error code could have come from any of a dozen of other problems.

How do you know the right solution? Back to the fever analogy, do you need surgery or an aspirin? Leave it to the pros.

2 Responses to “How The Check Engine Light Works”

  1. Cardoc3

    I like what you are doing here with the videos, but you have a few of the technical details “out of whack”. A flashing Check Engine Light, Service Engine Soon, or MIL as it is called occurs when the PCM detects a catalyst damaging missfire. Simply forcing air as alleged by the car wash drier into the air cleaner assembly, and then have the MAF (mass airflow sensor) provide an incorrect airflow to the PCM would first trigger a MIL for a fuel trim issue. Fuel trim correction should have prevented a missfire in that situation, but even if it didn’t achieving sufficient missfire rate to have the PCM flag a missfire while in that idle condition would first be an emissions missfire which would count up on the 1000 rev counter and likely set a P0300 if enough misfires were logged since it would not be a single cylinder event. Remember at idle, say 700rpm it would take almost 90 seconds to log that 1000 revs for the PCM to update the counter.

    There is also the 200rev counter which you will find is dependent on engine load. In order for a missfire to potentially produce “more” damage to the catalyst, a significant amount of unburned fuel, and air has to be reaching the catalyst. At idle each cylinder is drawing about 1/3rd as much air as it can at heavy throttle loads. That’s why the flashing MIL generally occurs under a heavier engine load, but could stop flashing at idle, or lighter throttle openings even though the cylinder is still misfiring.

    If you really want to push the envelope, we can start to discuss strategies such as PCM’s cancelling an injector pulse on a cylinder that it detects a missfire on. With no fuel being fed to a cylinder that was detected to be misfiring, it first can no longer hurt the catalyst, and secondly isn’t polluting the air. Given those two conditions, the driver will occasionally see the MIL flash, and then go back out with no code being stored by the PCM. We could do a four hour class on that strategy alone!


    I believe that Auto Net Tv has a potentially good concept here. This article on The CHECK ENGINE lite does bring to light a problem area, ACCURACY & PLAUSABILITY from the customers perspective. The article should address the situation from the drivers seat via educational facts from the techs. IE, Why is that lite on when I first turn my car on? What the heck is it flashing for {explain cat damaging situation}? What does it mean when it comes on while driving [elaborate on HARDWARE vs PERFORMANCE faults and codes].  Why did it go out after a few days [explain self checks, code catagories and drive cycles]. Explain too that codes may be present which "do not"  turn on the SERVICE ENGINE SOON lite. TOP THOUGHT, we the techs, as well as Auto Net Tv,  MUST, MUST interpret from the little operators living inside all these control modules what goes on, in TERMS the public is able to understand. We have to educate [NOT BS] the people in common terms, that  codes DO NOT isolate a failed component but address a problematic situation in a "SYSTEM". These systems basically begin at a module, communicate VIA  CONDUCTORS, to components or OTHER modules and receive conformation MESSAGES back to the originator, via another WIRE,  an EXPECTED response. When response agrees with command the little operators within the box agree and DO NOT turn on any SERVICE ENGINE SOON/CHECK ENGINE indicators.  My bottom line to the customer, The check engine lite is there to protect the enviornment. It points out a problematic situation which, if not corrected will allow tail pipe emissions to exceed limits and MAY lead to additional damages to the components under the hood. I need to confirm the code, analyze the data, isolate the "CAUSE" of the situation before an accurate estimate will be made. The time incurred to do just that does cost and in most cases can be incorporated into the final repair bill with, the FAILED ITEM delivered to YOU. Core charges, if applicable, will be assessed.


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