Hi everyone,
Today my topic isn't one that's rare, I just recovered my 03 S500 4Matic from the dealership after a transmission service. The mechanic had to drop the exhaust, thus disconnecting both rear O2 sensors to reach the transmission pan to replace 2 seals, the filter and refit the pan. On my way home, following the service, I stopped to fill the tank (it was about half full at the time). I left the car running, pumped my gas and left the gas station. about 30 minutes later, my check engine light came on. I called the dealer who had me bring the vehicle back in and subsequently told me that ALL FOUR O2 Sensors had malfunctioned. He had the mechanic recheck and came back exclaiming, "Good news and bad news. . . the good news is that all four O2 sensors were not burned out, but both rear sensors were gone." I asked him to reset the computer and give me some time to think on it since I had NEVER had these sensors go bad and it just seem coincidental. I neglected to tell him about the gas station refueling as I didn't at the time think it had anything to do with the problem. My question is, when the fault codes are revealed due to refueling while the engine is still running, are those codes the same? BTW I have been driving the car for a day now and no check engine light has illuminated yet. Any advice is graciously admissible, thanks in advance.

Don

 

Sorry guys clarification of the question, could the fault codes for bad O2 sensors be due to the incident at the gas station that I just described?

Don

 

Fault codes are exactly what their name implies. Codes. There is only one library of OBDII compliant codes + additions which are manufacturer specific.

The key question is what was the code ID. This points you towards the problem BUT does not necessarily tell you what is actually wrong. e.g. P0301 = Misfire Cyl 1.
It doesn't say faulty No. 1 plug or injector or coil etc

So why did you CEL Check Engine Light come on?

It is possible that the system sensed that the fuel system was no longer pressurised (cap off) whilst regassing with engine running. This would set a code.
The gas tank is pressurised when the engine is running and system pressure is measure to ensure system integrity.

Alternatively the "new" gas could be contaminated and this could have set a different code(s) indicating defective O2 sensor output / circuit or similar.

Refuelling with the engine running is not a good idea! Is it actually legal in the USA? Not so here in the UK.

Bad fuel would likely knock all 4 sensors out BUT this would be an EXTREMELY rare occurrence and would effect 100's of cars that regassed at the same station. Occurred here in the UK recently where many supermarket fuel stations were supplied with contaminated fuel and took out tens of thousands of car O2 sensors. Not enough sensors in the country to respond to the problem. delays etc. Quite a problem. The gas distribution company paid up!

Stuart

 

Thanks snanceki,
The gentleman at the dealership just came back to tell me that the CEL indicated the rear O2 sensors were gone. He didn't give me the codes, just told me he'd clear the computer at no charge and that I would notice poor gas mileage. In any event the CEL isn't back on; is it possible to get poor gas mileage due to bad O2 sensors even with the check engine light out. I thought the CEL was the indication that the sensors (and possibly other components) were gone.

-Don

 

Hi Don,

IMHO something is wrong in what the dealership has told you!

The rear sensors COULD be dead but this would likely give you a code that would re-illuminate the CEL.

In addition why would BOTH rear sensors go bad, at the same time, and not the front ones.
Its the front sensors that do most of the work and set the air/fuel mixture many times a second, that in turn impacts the gas consumption.

From the little info you have provided I suspect that all that was wrong was that you had the gas cap off with the engine running.
After a few engine start cycles the CEL would likely have reset itself automatically anyway.

I would cross examine the dealer and specifically ask for a HARD COPY report that shows the DTC codes linked to your VIN code with the correct date etc.

If they used a scanner without hard copy facility then I would wonder why they are not using MB approved Star SDS Diagnostics.

Revenue generation?

I think you were right to ask for time to think about this.

Why is it that there are so many off these "rip off" stories out of the USA.

IMHO MBUK are (unreasonably) mega expensive but at least are very professional.

Mmmm.

 

Well, I drove the car around all day today. . . no CEL. My primary concern is that I feel that the gas mileage has decreased; my assumption is that a quarter of a tank of gas is about 5.94 gallons, given that the tank holds 23.9 gallons of gasoline. I should be getting approximately 101 miles per quarter tank of gas give or take a few miles. I think that I'm getting between 80 and 90 miles per quarter. Does this seem a bit odd?(doesn't help that it's 20 degrees fahrenheit). Even with no CEL could both rear O2 sensors be highly "charged" enough to keep the CEL in check but too low to regulate air/fuel mixture for optimum mileage? Any advice?

Thanks,
Don

 

Something still wrong.

Bad gas mileage = bad tailpipe emissions. The engine ECU (indicate by the CEL) is designed to detect when the emissions are incorrect for legal reasons.

So how can this guy say the sensors are dead (system checks to see if they are working OK) yet the CEL is out.

Does the CEL come on when you first turn the ignition on. It should! i.e. He may have disconnected the bulb / LED or done it in software.
In this condition your car will not pass the annuyal smog test.

So what is (was) the code. If the light has been disabled the code should still be present.

Maybe I'm missing something?

To establish gas mileage you need to do a brimmed tank to brimmed tank test. Can't rely on the gauge!
Cold ambient + short journeys = poor gas mileage.
Maybe the garage has got you thinking about gas mileage, which of course has hit everybody hard in the recent past...so that you will take your car back to get the (unnecessary) work done.

Your O2 sensors may be bad but my original concerns exist.

Get the car re-scanned and witness the resultant code, or lack of it, for yourself.
No code = no (emissions) problem.
Poor gas consumption is then in the mind or as a result of the type/size of car.

I suspect that this MAY BE yet another scam!!

 

The CEL does come on to check when the car is initially turned on and it goes out just as it should normally. I do agree with you, I called the dealer today and asked for the fault codes but he couldn't tell me them right away, instead he said that there are several factors that can cause the sensors to trip and to hope that the CEL continues to stay off. I told him that I was advised (by you) that CEL doesn't spontaneously go out and not come back on, and that dead sensors would trigger the CEL right away. He disagreed, stating that sometimes there are several cycles for which the computer must go through in order to trigger this particular fault code. He went on to say that if the fault was due to engine running while refueling took place that the code would have INSTANTANEOUSLY appeared and would have read VACUUM PRESSURE (something or other). Anyway, the CEL remains off for now. By the way, the service manager said that the transmission fluid ran down the line of the sensors, which could have shorted them out. I looked at these sensors and they are insulated rather well, as they must be protected from the elements, I tend to disagree with him. He did say that he did not have the fault codes available (as he didn't charge me for the service) which makes me wonder if there were any codes in the first place. I will keep you posed SNANCEKI Happy New Year!

 

OK,
So the CEL function has NOT been turned off. Good.

I agree that some faults do not "immediately" illuminate the CEL and a few cycles might be required.
This is so as to desensitive the system to "spurious" warnings.
The ECU knows however that a fault has occurred and sets a code (pending) but doesn't illuminate the CEL.

IMHO rather than a number of cycles (which as I say above can sometimes be required) I would favour a specific set of conditions needing to reoccur.

Give the car a bit of "exercise". See what its performance is really capable of! CEL come back on?

HOPE is not a term I agree with.

If the CEL comes ON it signals a code has been set that indicates a fault that impacts emission certification has occurred and that a cause for this exists and needs to be rectified.

I agree that if the gas cap was off you would get a code that indicates tank pressure low, or similar, rather than an O2 sensor code but your mechanic doesn't seem to remember what the code was. Mmm.

WRT trans oil going down the harness.
Yes this is a known MB issue which is easily rectified without draining the trans. removing the pan. Connector is above fluid level.
Just unplug the electrical connection to the trans and fit a new adaptor complete with new o rings (different material spec. introduced).
The extent of this leak is more often dampness / dirt discolouration rather than a real "leak".
Its usually a keep an eye out type job.
Good revenue earner
However if there is a really "good" leak this can impact the wiring but I'm not sure how soon the O2 sensor wires join the trans harness.
Certainly the O2 sensor connectors shouldn't be swimming in oil. Note: Oil is an insulator rather than a conductor.

Good luck. I suspect you will get the CEL back on once those conditions reoccur.
When this does happen make sure you witness the codes set or get a hard copy.
That way everybody can move forward on an agreed basis.

Happy New Year.
Stuart

 

So Far to this date, no CEL illuminates since I last reported the problem on 01-01. Stan, I am sure you hit the diagnosis well on the head, I believe in town I get around 100 miles through a quarter of a tank, which equals around 16mpg and some change. still stinks! but around 22mpg highway as I traveled to Washington D.C. for the inaugural address. Total from full tank to about the beginning of last quarter of the tank was around 287-295 total miles. Stan you're A-1 in my book thanks for helping us all!!!