Check engine light shuts itself off - meaning?

Squeaky · Post by **Squeaky** » 16 Oct 2006, 23:56

Hi folks,

Back with more questions about the check engine light on my '94 850 turbo wagon.

I'm still trying to figure out why it keeps coming on - still no sign of the promised help from my dealer.

My latest question: several times now the engine check light has shut itself off - what does this mean?

Squeaky

Volgrrr · Post by **Volgrrr** » 17 Oct 2006, 01:05

It could be a variety of things.

The surest way is to check your fault codes from the diagnostic boxes A & B) located just behind the RH headlight.

You will find out how to use these diagnostic boxes as well as understanding the meaning of any stored fault codes elsewhere on this excellent sight.

If the fault has gone, the fault code can be cancelled - if the fault is still present then the indicated fault code cannot be cancelled.

Bear in mind, if the Electronic Control Module considers the vehicle is running, say for example, either lean or rich at any point in time compared to the parameters programmed into the computer (e.g. maybe after some hard driving), the ECL will be switched on.

The computer is programmed to run a series of check cycles after finding a discrepancy, to establish if the fault is still locked on. If the fault reoccurs during that time - the computer kepps the ECL switched on.

I don't know how many cycles or how long the cycles take, but it might be say something like the equivalent of 10 minutes elapsed driving time (the real experts on this sight would know this information).

However, if the fault is fairly infrequent, the computer will go through its series of cycles - not find a fault, assures itself everthing is back to normal and switches the ECL off.

Let the diagnotic boxes be your starting point and work your way along from there in a methodical, logical manner.

Squeaky · Post by **Squeaky** » 17 Oct 2006, 19:55

Thanks Volgrrr in Victoria, Australia

I appreciated the input.

Actually, the codes have been read at least a dozen times in the year and a half that I've owned the car. The knock sensor has been replaced twice (the last time included the new updated wiring kit), but the codes continue to indicate a knock sensor problem.

The ECL stays on much of the time, but once in a while it goes out for a few hundred miles or less. Would you say that I am correct in assuming from your explanation that it would not go off if the diagnostic process had not told the computer that the fault was at least temporarily corrected?

By the way, there is a third type of car owner - one that owns a Volvo, but wishes that they came with more reliable and less frustrating ECL technology

Squeaky from Victoria, Canada

Post by **MadeInJapan** » 17 Oct 2006, 21:49

What octane gas are you using? Could just be low or you're buying some sub-par gas.

Squeaky · Post by **Squeaky** » 18 Oct 2006, 00:55

Hi MIJ,

I'm using 91 octane - tried 94 for a while, but it made no difference.

Is there a minimum level for that engine? Is it a case of the higher the better?

Thanks,
Squeaky

Post by **MadeInJapan** » 18 Oct 2006, 12:34

Turbo wagon- recommended (from your owner's manual) at least 91, so you should be okay. Just checking because low octane rated gas could cause pinging, and thus kick on the knock sensors.

Volgrrr · Post by **Volgrrr** » 20 Oct 2006, 21:16

Hi Sqeaky from Victoria, Canada,

Yes, if what I've read is correct, the computer runs through a predetermined number of cycles and if it has not collected any more fault inputs then it sees the fault as cleared. It gets a bit tricky if the fault is intermittent.

My ECL was going crazy for a while - I took it in to the workshop, they mucked about with it and achieved nothing (however, just because they couldn't find the fault didn't mean they did it free of charge).

So I decided to put some injector cleaner in the fuel on the 'nothing ventured, nothing gained' principle.

Nothing appeared to change over a few hundred kms, then one day - hey presto! I noticed the light was off - and it has stayed off ever since (touch wood).

Coincidence?

It might be, I really don't know - but right now I'm perfectly happy with how everything has turned out.

ferrari9 · Post by **ferrari9** » 22 Oct 2006, 21:36

Squeaky -

Your Volvo's ECM stores diagnostic trouble codes (DTCs) differently in accordance with a pre-determined pattern. An emission-related fault must be present for a pre-determined number of intervals before your ECM permanently stores the DTC & illuminates your Malfunction Indicator Light (a.k.a. Check Engine Light, a.k.a. Lambda Light, a.k.a. MIL). If the fault has cleared by the time the Volvo-specified interval has passed, then the DTC will be erased from it's memory.
There are 3 main types of intervals:
1. Trip; all of your car's diagnostic functions have been performed.
2. Running Cycle; Car's Fuel Trim (closed loop) has been initiated.
3. Time (time in seconds)
If the fault disapears for any reason after the DTC has been recorded permanently by the ECM, the DTC info will now be retained by the ECM. Each time the fault occurs, the 1st counter will advance one (1). On each trip completed without a reoccurrence, a second counter will count down from three (3) to zero (0); when the second counter has reached zero (0), your CHECK ENGINE light will disapear from the screen. At that very moment, a third counter will begin. This third counter keeps track of the number of 'warm-up cycles' starting with 40. A 'warm-up cycle' is when the engine temp is less than 104 F (40 C) at engine start and exceeds 183 F
(84 C). If the third counter reaches zero (0), the DTC is erased from the ECM's memory. If a fault then reoccurs - the 2nd & 3rd counters then revert to their default settings and the 1st counter advances one.

...taken directly from the 95 850 Service Manual (found right here on this beautiful website

)

I hope this helps... I know this is the complicated version, but I figured I'd give you the complete analysis procedure for the ECM and you'll figure out everything in between.
If you have any questions understanding the preceeding information or what your codes mean and why, don't hesitate to ask... there are many fantastic enthusiasts around to help!
Oh and also... I have read that this warning lamp can be caused by faulty vacuum hoses... not sure how accurate that is but I am looking into it at the moment...