1998 Cadillac Seville AX 1500 Code Scanner Reading Codes

Ok before going to deep I would clean the maf with maf cleaner and check that #3 plug then clear codes and drive it see how it runs. If the engine light comes back on the run another test. If you do not clear the codes the old ones will stay until cleared weather it is fixed or not. Let me know how it goes ok.

It's not my car and I cannot work on it until next week and need this info now to get ready. I am helping someone with their car so it can pass inspection and to do amy repairs that I am able to.

I already got all that info and alot more from a previous tech with a very fair donation given.

My main question is more related to this:

I added the oil after the check oil light went on. A short time later driving around the engine light came on for the 1st time and then went off after a few days of driving.

1 week after the light had gone on I read the 3 codes mentioned. The P0303 was a pending code, the other 2 were current.

Why did the engine light come on after adding the oil and then go off a few days later?

Why did the engine light go off after a few days of being on after adding the oil and not go back on for weeks of driving around with the 3 codes having already been read at the beginning of those 3 weeks of driving and now it's back on starting 1-2 weeks ago?

Why hasn't the P0303 changed from a pending code to a current code by now and if it has why wouldn't it's new staus show up on a new read without clearing the ecm of the 3 already read codes?

If another new code was to show up on the ecm wouldn't that show up on the code reader if I did a new read without clearing the 3 already read codes from the ecm?

That's why I am asking you, why or when should the P0303 pending code show as current?

Are you telling me that if a new code was to be on the ecm that the code reader couldn't read it because the ecm will not give a new report until all the codes that were read on the prior read report are cleared?

What about a new 4th code that wasn't on the last code reader report from the ecm. Wouldn't that 4th new code show up on the reader in addition to the 3 previously read codes without clearing the 3 codes from the ecm first?

It doesn't make sense that the ecm or scan tool would be so limited because if a new code shows up or if a pending code changes to current it should be able to read the ecm's new staus without clearing the other codes.

The new code should appear along with the 3 previously read codes and the new status of the pending to current code should be there too.

The question I am asking is:

Will the ecm tell the code reader that a pending code has changed to current without clearing that same pending code?

If a new 4th code was now on the ecm would it show up on the code reader if the 3 prevbiously read codes were not cleared from the ecm first?

I should have asked these questions more directly the 1st time.

Ok first off yes the new codes would show up if there is any new ones to be recorded but if the light came on because of one of the previous codes then it will not set it again it is already there. Also you clear the codes so the computer know's that they are gone. IF a code is still there the computer thinks the codes are still problems and runs as such. The other reason you should clear them and then drive it till the light comes back on is so that you can see which codes come back so you better diagnos the car to fix it. As for why the light came on after adding oil makes no sense that should have had nothing to do with it. I am just trying to help you and info is very cridical for me to do that so the more info you give upfront the more that helps me and better I can help you. As for a misfire affecting the cat is may make it run hot as for it being pluged the only way to really tell it to have an Muffler shop check for exhaust flow. I hope this was more of what you were looking for anything else just ask. Thanks

Very good,

I'll zero in on what will help me with this project and using the code scanner to work with the ecm.


Ok first off yes the new codes would show up if
there is any new ones to be recorded:

Just to be sure, are you saying that, yes, even if the ecm is not cleared of the 3 already read codes, that
any new codes would show up during a new read,
along with the 3 codes from the previously read, that have not been cleared from the ecm?

Would a current P0303 code read out from the ecm also, without clearing the P0303 pending code first from the ecm.

Is a pending P0303 ususlly a slight or intermittent misfire?

Is a current P0303 a more regular misfire?

Does a pending misfire change to a current missfire on the ecm even if the level of the misfire stays the same, after a certain amount of driving time?

What does a pending P0303 say about the 3rd cylinder misfire, and how does it compare to a current P0303, as far as being minor, major, spark plug, or fuel injector, etc, as the cause.


As for a misfire affecting the cat it may make it run hot as for it being plugged the only way to really tell is to have a Muffler shop check for exhaust flow:

How long can a cat be run hot before it starts to clog up or not be able to pass it's readiness test?

Is this right? The overly high heat degrades the catalyst, so the unburned fuel or carbon deposits start to clog up the cat.

Will A cat throw a code if it is running hot?

Will A cat throw a code if it starts to degrade?

How many miles of driving with a pending or current single cylinder misfire will start to plug or really plug a cat?


Also you clear the codes so the computer know's that they are gone:

Do you mean after the car is repaired?

IF a code is still there the computer thinks the codes are still problems and runs as such:

Are you saying, that the ecm tries to help the
engines performance by changing settings here and there, etc, etc to compensate for the codes it has picked up?

If yes than wouldn't it be better not to keep clearing the codes if repairs cannot be done right away or soon because the ecm changes settings for better pereformance with the conditions that the codes represent

and/or that the ecm will slow down performance to protect the car with the conditions that the codes represent?


The other reason you should clear them and then drive it till the light comes back on is so that you can see which codes come back so you better diagnos the car to fix it:

I got this part about clearing the codes 1 time so the ecm can check whether any of the codes have gone away even if no repairs were done yet, and that most likely most or all of them would be set again if no repairs are done, unless they were set off by driving way out of the normal type of driving pattern which the ecm is well aware of, which could trigger a code, that would not have been set off during a regular or more normal driving pattern.

Do all engine codes set the engine light? if not, why not?
What kinds of codes set the engine light?
Does it really matter, because it's only telling you that there is an engine code.


As for why the light came on after adding oil makes no sense that should have had nothing to do with it:

This is what happened: I was driving this car after it was running for 5 minutes and and it started to completely misfire and quickly lurch forward running again. It did this pattern a few times in a row and then started running normal again for a few seconds and then did the pattern again and then it started to stay running normal.

Later I checked with the owner, this had never happened before.

I very quickly parked the car and noticed that the check oil light was on. I shut the engine off. I had to add 2 quarts of oil to fill it all the way to max. I did not go over the max line at all.

Later on I read on a Cadilac forum that on that engine it is better to only keep the oil filled up to the top of the fill line and not at the top of the max line.

I told you that the engine light was also on, but now I'm not sure if the engine light was also on at this time.

Because the oil was 2 quarts low, after putting in the Lucas and the 2cnd quart of oil I wanted to be sure that the engine was ok, so I was using the overdrive alot to check for its normal race car like performance. It was all local driving so I had to keep slowing down.

At first the tranny was not shifting smoothly and the car was sluggish or hesitant.

After a while the tranny started to drive normal with full overdrive etc.

After driving around for awhile the tranny was shifting ok and the engine had full power but it wouldn't kick into overdrive.

So I parked the car and the engine was very hot.

The ecm might have been telling the tranny to stop using overdrive?

During this hard run might have been when the engine light was 1st lit and not at the same time that the check oil light was 1st lit.

Later I read on alot of Cadilac forums that this engine does better when it's run hard once in a while or even better if done regularly on a highway or open road, I'm not sure about alot of hard short runs around town with stop signs and traffic lights.

The next day and since then the car runs good and with lots of power. It's down 1 quart of oil just where it's been said it should be kept at.
Less than 1 week later the engine light went off. A few days later I used the scan tool and got the three codes P0100 current, P0101 current, P0303 pending.

Why did the engine light go off if no codes were cleared?

Can the engine light go on without a code being set?

Can a code be set without the engine light going on?

3 weeks later I was zipping around in the car, but not nearly as fast as the
1st run mentioned above and the engine light was on again.

On my next code read there were no new codes, so why would the engine light go on again, without a new code being set?

It's been on since then, and is still on.

I used the scan tool recently and the same 3 codes are there and the P0303 is not showing that it has updated to current.

All of the 8 readiness codes were passed on the code reader on the 1st and 2cnd read.

I've never worked on a late model car before, so the more you can answer from the above the better, before I start taking things apart.

I'll clear the codes and drive it around before I work on the car.

Thank You

Ok first you keep pulling codes right? You have no new codes because one of the already set codes is causing the check engine light to come on. See if you leave the codes in the computer it will not set new codes if it is one of the old ones that are still present. As for the pending code of the miss fire tells me that it doesn't missfire all the time this is possibly what is causing the light to come on and off it will not set a new code because it is already stored. AS for what is causing the missfire it could be a plug a plug wire or injector. You will not know if it is the injector without a noid light and it missing at the same time. With that I would put a can of seafoam in the fuel and clean the mass air flow meter with mass air flow cleaner. I would then clear the computer codes and drive like normal. See if you drive hard and race around everywhere the computer adjusts shift points and throttle and many other peramiters to your driving so if you then drive like grandma the computer readjusts to those habbits and then you go back to racing around things seem like they are slugish it will the computer about 10 miles to relearn your new driving habbits.
As for a light coming on and not setting a code yes it can happen. As for why it all depends on the conditions for why it came on in the first place. If it picks up a lean condition and it is for a very short time then the light comes on and goes back out if it see's the condition long enough it will then set a code.
Now as for the oil light and engine light to come on together would be because the oil is low the low oil sensor see that turns on the oil light and then turn on the engine light to get your attention that hey without oil I will die and leave you stranded. As for being low on oil you need to know where it is going is it leaking is it using it or what. IF it is using it then I would do a compression test to see if there is a low cylinder.
As for the cat. If you run it and it is red glowing red then you have lots of fuel in the exhaust and you will burn up and plug the cat. As for how long it can take a rich condition depends on how rich it is on an occational missfire it should not harm the cat to the point of having to replace it. As far as the muffler shop you only need to see them if you suspect the cat is plugged so they can do a flow test and tell you if it is plugged or not. From what you have now told me I would say this is not a problem as it will run good when it warm. When a cat goes bad it will run fine till it gets warmed up and then will loose power like you are pulling a heavy load. I hope this better explains your questions if you have any more just ask and I will do my best to answer if I don't understand what you are asking then I will ask to better explain ok.

As for the pending code of the miss fire tells me that it doesn't missfire all the time this is possibly what is causing the light to come on and off it will not set a new code because it is already stored:

Why would the light ever go off if the code that brought on the light has not ever been cleared?

Even if the misfire does go away, doesn't the ecm continue to react as if the condition is always present if the code is not first cleared?

In most cases that is true except for live data which a missfire is live data so the light comes on when it is missing and goes out when it quits. Therefore it won't set a code because that code is already present.

The engine light was not on when i did the first read.

The light has been on for 3 weeks steady and is still on, so why hasn't a current P0303 showed up if misfires are always read as live data?

There have not been are there are no new non live data codes set that would cause the light to stay on steady.

Because it has to be missing at that time that is why it is pending it not a continual missfire it is interemittent so it is pending. One of the codes is causing it to stay on so clean the mass air flow unit with mass air flow cleaner and do the seafoam in the gas tank clear the present codes and drive it so you can see what code is coming back to cause you problem and the light to come on.

Thank you for telling me about Seafoam, it sounds like a great oil and fuel additive that really can get alot of these kinds of repairs done, instead of having to take things apart to clean them and by the fuel performing so much better.

I have always used the best additives I could find to try and fix certain auto conditions.

Lucas HD Oil Stabilizer is highly effective at protecting and getting better performance for engines and helprng to restore older engines just by adding it to your oil normally at 20-25% and up to 60% for badly worn engines.

I will clear the codes and do the repairs as you said.

Why was there no light on when I did the 1st code read 2 months ago and at the same time I found the 3 codes P0100 C, P0101 C, and P0103 P.

There was no light on for 2 weeks after that code read, then the light was on steady for 3 weeks up until now and is still on. I did a code read and there are no new codes set.

Should I add seafoam to the oil as well?

I have a bottle of Lucas fuel additive.
Would Lucas fuel additive do the same as seafoam in the gas?

What about the product called k100-g fuel treatment? It sounds similar to seafoam.

Which is better?

Can they both or all 3 be used together in the right ratios?

Could any of the 3 above also clean the #3 spark plug without having to remove the plug?

About cleaning the MAF sensor:
I read the post below on a Cadilac forum for the same car about how to clean the MAF sensor.

Does the MAF sensor have to be removed to clean it as it says in the post?

Should I also remove the screen that the poster says was removed for better performance and economy?

Cadilac forum post:
As for the MAF Sensnor yes spray it with MAF Sensor Cleaner. The sensor is screwed on to the throttle body. You must remove it to clean it as the front of the sensor has a screen, which i removed for better performance and Economy. Try that before replacing the sensor itself.

No do not use all three products at once just one. I prefer seafoam you use what you like ok. As for the codes being there with no light they we set at one point and never cleared. As for the mass air flow unit yes remove it and use only mass air flow cleaner and reinstall it. Then clear the codes and see if any come back. As for #3 missfire you are going to have to pull that plug check it and clean or replace if it is fouled.

I haven't workled on the car yet, or cleared the codes. So, why did the engine light go off?

Does the engine light go out after a while if no new codes are set?

Should I also remove the MAF screen permanently that the poster says was removed for better performance and economy?

Does he mean that he removed and did not replace the screen for better performance?

Cadilac forum post:
As for the MAF Sensnor, yes spray it with MAF Sensor Cleaner. The sensor is screwed on to the throttle body. You must remove it to clean it as the front of the sensor has a screen, which I removed for better performance and economy. Try that before replacing the sensor itself.

Does he mean, that he removed, and did not reinstall the screen after cleaning the MAF sensor, for better performance?

In other words remove the MAF sensor, remove the screen, clean the MAF sensor, check/clean the electrical contacts, reinstall the MAF sensor, and do not reinstall the screen.

Do not reinstall the screen for better performance and economy? Yes, no, maybe??? What is your opinion on removing the screen permanently?

Or did he mean to remove the screen to do a better cleaning of the MAF sensor, for better performance and economy? And then, yes reinstall the screen along with the MAF sensor?

First I am not going to get into it over another post at another site. As a mechanic and ASE certified I would just clean it and leave the screen in place it is there to keep larger stuff off the sensor. As for the light it no longer see's the problem so light out. Good luck