Does Check Engine Light Reset Itself?
 

Recently, the check engine light on my car has come on starting every few weeks, and now every day or two. Each time, I would count back the blinks (5 blinks), and then reset it, hoping that I would get to the mechanic soon to get it checked out. Over the phone, he told me it was nothing serious, and that I should just bring it in when it was convenient. Well, I reset it about a week ago, and sure enough, it came back. Since I planned on going to the shop in the next few days, I decided to not reset it, so that any stored codes would be available for the mechanic. Today, through no intervention of my own, the light turned off. Thinking that maybe the bulb burned out, I turned off the car, and turned it back to the on position, and it lights up. As soon as I start the car, it is gone. I then checked the computer to count back the blinks, but there were no stored codes. Does the check engine light reset itself after so many start up/shut down cycles? What else could have caused the check engine light to reset itself? Thanks in advance.

By the way, I have a 94 E420.

The check engine light resets itself on newer OBDII cars... I believe starting in 98, maybe 97 or 99... So yes, it does on newer cars. And no, to the best of my knowledge, your 94 E320's check engine light does NOT reset itself.

HTH

Virtually ANY car that has a check engine type light will reset the light if the trouble goes away. This does not, however, remove the trouble code, it will still there to be extract and read until the codes are deliberately cleared.

Have a great day,

For your '94 E420. . . .
 

it does have a malfunction counter. This counter is decremented by one count for every 'trip'. A trip is defined as KO, running at great than 5mph and longer than 5 minutes (if I remember the numbers correctly).

So if the CE lights and for next 'x' trips if the DTC doesn't occur, the counter is reduced by one. So that's how the light goes out.

5 blinks = EGR recirculation faulty. Probably plugged!

Thanks for the replies. Would someone please explain how and where to check the EGR?

May be plugged , but the light usually stay when it is..
There is a TSB for sticking EGR valve with a part up-grade for this car. That is more likely the problem considering the lamp goes out [ indicating intermittant prob.]

Check Both...

The EGR on that model is at the end of the exhaust manifold.
The easy test is to apply vac [ with a hand pump] to the small inlet of the valve while the car is at idle.
If the car run rough when the vac opens the egr and the egr snaps shut when you release the vac., the egr is suspect of sticking .
If the valve opens and no change in idle, the pipe at the intake from the valve is plugged [ common on 104 eng]
* see archieve DIY article by SB.