Check engine light "on" -vs- "flashing" what's the difference?

Question

Quite often, people inquire as how to "clear check engine light".

Most of the answers are simply to disconnect the battery for a period of time. (myself included)

So, my questions are:

1) What causes a "check engine" light to come on?
2) Do some come on after a certain point in time, or mileage as a reminder to have the engine serviced.
3) If, someone simply wants to reset the computer themselves, why is it wrong to advise them to do so?
4) If you had a check engine light come on if you simply had an engine misfire because of some water in the fuel, will the check engine light turn off by it's self?
My thought is, if it were the friendly reminder type, it should remain off. However, if it reappears, then you should have the computer codes checked for possible causes.

Now for "flashing" check engine light.
I understand that means there is a dangerous condition in the engine that needs imediate attention. (right?)

Answer 1

A flashing MIL, means there is a 'class A' misfire that is present. This type of misfire WILL ruin the catalytic converter(s), if the problem isn't fixed in a timely manner.

Answer 2

The logic behind the OBD II system that determines a fault to exist in the monitored emission control systems is defined in the "Bible of OBD II" which is the Title 13 of the California Code of Regulations (CCR) 1968.1 That document tells you anything you would want to know about what is monitored, how it is monitored and the test criteria that is required to turn on and turn off the MIL (Malfunction Indicator Lamp) that is commonly called the "check engine" light.

The light comes on when the OBD II system determines that there is a failure in a system or component that could lead to excessive emissions. Excessive emissions are defined as emissions in excess of 150% of the FTP or 'federal test protocol' and are unrelated to state smog check criteria. The OBD II system monitors for both component failures and system failures.

a component failure is detected when a part doesn't function as it should. The tests for components are tests for normality (is it operating in the range that a normal sensor or component should operate in) and rationality (does the reading from the sensor or the action of the component make sense when compared to the inputs from other sensors. For example, if a sensor has a 0 volt input signal to the PCM, the sensor is open circuited and outside the 'normal' range of operation hence a normality failure. A rationality failure is when the vehicle speed sensor says the car is going 50 miles per hour, the crank angle sensor says the engine rpm is 3000 and the throttle position sensor has 0.5 volts indicating a closed throttle. If that condition persists over the test time, the system says that the throttle position sensor is not working correctly because the engine cannot continue to run at that speed with a closed throttle and the OBD II system determines there is a rationality fault.

On most OBD II components and systems, the fault must be initially detected and confirmed on a second test before the MIL is illuminated. The testing on some components is continuously monitored. Others are tested only once after the vehicle meets test conditions. Essentially those conditions include a change in engine temperature of at least 70 degrees F, and elapsed time, vehicle speed and engine load conditions that must be met before the test begins. The OBD II tests are not run every trip. Most monitored conditions must fail two consecutive tests for the MIL to be illuminated. If the vehicle is then driven for two consecutive trips and the problem is not detected, the light is shut off. A loose gas cap is an example of that--if the cap is loose, the light is triggered after two test failures. Tighten the cap and after two test sequences the light will go out.

The exception to the two test logic is for conditions that will damage the catalytic converter such as a dead misfire. For those conditions the MIL will flash continously on the first time the misfire is detected and stay flashing until the misfire is corrected.

2) The MIL has nothing to do with service intervals, fluid levels or maintenance.

3) Because of the testing that is done to determine that a problem exists, the issue of reseting the computer is moot. You can clear a code, but if you don't fix the cause of the MIL the light will come back on again and again until the cause is repaired.

4) if your misfire was water in the fuel or other temporary condition the light will go out by itself when the engine is no longer misfiring. this may take a few trips to shut the light out. I've seen cars where the owners have filled the gas tank with diesel fuel causing the misfire. purging the diesel fuel and refilling the gas tank then driving the car was all that was required for the light to go out. You can clear the code with a scan tool then drive the vehicle to confirm the repair using the MIL tests as your confirmation of the repair.

sorry if this is too much information. if you want more info on all fo the tests for all of the components then look up Title 13 CCR 1968.1 Its rivoting reading

hope that helps

Answer 3

it could be any censor in the engine,
to reset the ecu you will need to unplug the battery for a couple minutes then the check light should come off,
whenever it blinks a certain number of times, it is blinking a certain code for a certain sensor or problem

Answer 4

Oil, water, heat are the usual suspects.
If the light stays on it is time for a mechanic!