I'm getting some strange fault codes on my 2000 S500. Anyone else experience Fuel trim and oxygen sensor fault codes at the same time. Sometimes I get engine misfire fault codes also. They stay off for a day or so after I clear the codes and then come back.

Any suggestions?

How many oxygen sensors are on this care and where are they?

Thanks,
dm

 

Quite straightforward really.
You have 2 O2 sensors per bank. One on the engine side and one on the downstrean side OF THE Front cats. There are two cats per bank.

Most of you problem appears to be on bank 1 / cyl 1.

You have the classic P0170/0173 MAF signature BUT since you also have O2 sensor codes I'm not sure whether the O2 sensor or the MAF would be the first port of call.
I would go with the O2 sensors (although I'm always reluctant to change them) since the misfire and MAF issues are only on Bank 1 . Since the MAF is common to both banks a faulty MAF would likely give fuel ratio out of range problems on both banks.

Note: The MAF and O2 sensors work as two parts of the same system. The MAF determines the base mixture for the given conditions and the O2 sensor checks to determine whether it is correct (many times a second) by determining the amount of O2 in the exhaust.
If the O2 sensor isn't working correctly then it can cause the MAF to go out of range.

The most likely O2 sensor to have gone down is the one nearest the engine on Bank 1.

Ensure you use genuine Bosch / Mercedes spares.

Stuart

 

I turned on the car and hooked up the reader and the current is steady on oxygen sensor 2 but is bouncing on oxygen sensor 1. I think I should replace oxygen sensor 1. What do you think?

thanks

 

Hi,
Depends on what you are using to determine that the current is "bouncing" around.

An appropriate mid range scanner will graph the sensor VOLTAGE against time as signalled to the ECU.
The voltage on the upstream (near engine) sensor should be in the order of 1 volt whilst the rear sensor should be much nearer to 0. The signal should also be cycling several times every second. This signal (voltage) is what changes the fuelling thru the ECU and then the injectors.

Sensor 2 voltage should be low since sensor 2 is only used to ensure that the precat and sensor 1 are working correctly. If sensor 2 sees any significant voltage it then highlights that "something is wrong" and signals a code that the precat is "not working effectively".

The gearbox cat is 90% a safety net (+ is used during difficult warm up conditions etc) and its effectiveness / output is NOT measured / fedback to the ECU (No sensors to do this) EXCEPT when you go for a smog test. The smog test sniffs tailpipe emissions.

I stand by my previous post and am not sure whether it is the O2 sensor or the MAF. However since you have a code specific to Bank 1 Sensor 1 O2 malfunction I guess that this is the more likely root cause since the MAF is common to both banks.

Hope this helps.

Stuart

 

I checked the oxygen sensors with the autoxray unit.

It shows as follows:
Oxy Sensor 1 bank 1 alternates between .150 v, .470v, and .735 v

Oxy Sensor 2 bank 1 steady at .480 volts

Does this add anything to the equation????

I also went tot he next screen and it showed Oxy sensor 1and 2 bank 2 and it gave me similar results. I have not checked this before. I assumed that the bouncing that I described was the problem, but if bank 2 is doing the same thing perhaps the Oxy sensor is OK and its something else. I've been using regular gas. Did this screw something up?

Is bank 1 the drivers side or the passenger side?

Is there some other information from the scanner that would be useful to properly diagnose this?



thanks,

Dom

 

Yep sensor 1 voltages sound OK (from memory) but not so sure about sensor 2 without checking. From memory I was expecting it to be lower. will check when I have a moment or two.
Bank 1 is the RH when sat in the car.
The key bit of missing data is the "refresh" rate of the signal. It should "bounce around" in your terms several times a second in a regular waveform. You need a graphing scanner or scope to see this.
You should use the recommended gas.
Not sure but Regular in the States is something like 87 RON (engine is designed for more like 95RON) which although OK in an emergency (mixed with some better fuel) is not OK long term and a false economy because the engine applies a lot of retard on the ignition which in turn reduces power and fuel economy.
However I don't believe it should kill you sensor although the slightest trace of some contaminants in the fuel will kill the sensor. e.g. silicon.

 

The car feels like it is running fine and I would never had known if not for the check engine light.

Its a 2000 but it only has 32K miles on it. What typically goes wrong first?

Should I first try replacing the MAS or the oxygen sensor or could it be a third problem?


Dom

 

Or since its running fine should I fill it up with super and forget about the check engine light and these trouble codes?

Dom

 

You should use the grade of fuel recommended in the handbook (I assume that this is "Super" in the USA).

The codes should be cleared and observed when, how quickly they come back.
IF the codes come back this means that something IS wrong (/ out of spec) and should therefore be rectified EVEN IF you cannot detect the fault under normal driving.

Changing the fuel COULD make a difference but its a (very) long shot based on the codes you are displaying.

You will not pass your annual smog check (assume USA) IF the yellow Check Engine / MIL / CEL is ON.

Stuasrt.