I have owned a 1995 E320 ( 4 doors sedan, 98k miles) since Nov 07.
About 4 months ago, the “ check engine light” came on, then I brought my car to a local shop and asked mechanic check it for me. He used the machine and found and wrote me the codes:
02 O2S HFM- SFT
12 O2S Heater Sensor 145
03 O2 control HFM
And he told me that the oxygen sensor need to be replaced, and he turned the “ check engine light” off. Two week after finding, I brought my car to another shop and replaced the O2 sensor bye the new Bosch one.
Then my car had been running good until yesterday, the “ check engine light” comes on again. In this morning, I brought my MB to find the codes, and the mechanic showed me that he had found almost the same codes like the last time.
Codes: 12 O2S Heater
02 O2S
04 Secondary ………( I can’t read this code)
The mechanic told me that the oxygen sensor needs to be replaced again, and he also told me that there might be some problem with the Catalytic Converter so this might cause the burning of the O2S. Therefore I need to replace both the O2S and the Catalytic Converter, and I think it would cost a lot of money.
My questions are:

Is that what the mechanic said about the Catalytic Converter is right?
How much it cost to replace these parts, and where can I buy the Catalytic Converter?
How it would effect my car if I don’t replace it?[/ol]
I would like to receive some recommends from youguys. This is the first time I have owned a MB so I don’t have much experience.
Thanks so much.

 

Codes do not actually tell you what is wrong. They just point to the system that isn't behaving correctly.
However in this instance the codes do suggest that the O2 sensor is faulty but the code that you couldn't read is most likely Secondary Air Injection fault.

Additional tests (O2 sensor waveform and output) should be undertaken to confirm that the O2 sensor is dead. If so the question why must be explored. WHY. How many miles since it ws changed.

You don't actually give mileage / service details etc so I can't comment but any number of things can Kill an O2 sensor. Bad fuel, contaminants from engine repair work, excess oil consumption and I guess a defective upstream cat might also cause problems but wouldn't have expected it to have caused the O2 sensor to fail, just change its output to advise the ECU that fuelling is incorrect. However this does not confirm that your cat is OK.

The problem with a CEL ON is that you won't pass the SMOG test when it is next submitted.

I SUSPECT that in fact you have other faults upstream of the O2 sensor since the O2 sensor only reads the results of upstream activity and provides feedback. This is why you need to know whether the output voltage on the O2 sensor is within range and whether the sensor is cycling correctly. If this test confirms the O2 sensor is dead then you need to ask why. Likely contamination or excess oil consumption. Incidentally what is your oil consumption?

Will be interested to see what others say.

Stuart

 

I have changed the O2 sensor for about 5k miles.
Since I bought my car, I have always used the premium fuel and the mobil 1 for the oil ( the local shop recommended me that the mobil oil would be good, but i don't know is it aright oil for the mercedes).
Do I have to bring my car to a local mercedes dealer to test for the reason?I have asked several shops but they don't work on the 1995 mercedes or later.
Thank you for your helping.

 

If the car has been OK for 5k miles it appears that the O2 sensor was wrong when it was originally changed.
How long did the CEL light stay off for after the first garage turned it off. Up to when the new sensor was fitted or did it come back on again very quickly?

It is uncommon for a sensor to go so quickly unless something else is wrong. Sure it was the correct type?

What codes are present now. EXACTLY the same as before? Please confirm.
What is the oil consumption on the car?

I missed it first time around but the first code on your original list is the Hot Film Sensor MAF (HFM - SFT). This controls the fueling which in turn is checked for being correct by the O2 sensor. So as I stated previously it may be the MAF that is giving the wrong fueling which causes the correctly operating O2 sensor to turn the CEL on.
The O2 sensor and the MAF (plus other components) work as a closed loop system (i.e. feedback).

Not much else I can do without some answers.

stuart

 

At the first time, after the machenic turned the CEL off, it had never come on until it was changed. But at this time, it came on againright the day after it had been turned off.
The machenic ordered the O2 sensor for me so i think it should be the correct type.
There are two of the three codes present now are the same as before ( 12-O2SensorHeaterand 02- O2 Sensor HFM SFT), and one codes is different ( 04-Secondary Air Injection now and 03-Control HFM before). The oil comsumption on the car is good.
Thanks.

 

that model year motor 104 has issues with deteriorating engine wire harness, and can cause
issues with numerous things. the insulation on the wires becomes very brittle and exposing bare wires together.

 

Mmm.
So CEL came on. Dealer reads codes and turns of light but does nothing else. Car runs OK for 5k miles. You decide to have a new O2 sensor fitted and within one day the CEL is back on with very similar codes. Correct?

Did the CEL come on whilst you were accelerating hard?

So why did you change the O2 sensor?
I'm not convinced from my understanding that it was faulty.
IMHO O2 sensors are changed as a first gut reaction when in fact the sensor is not faulty. Even when not faulty fitment of a new sensor can temporarily fix the problem since the new sensor will have a different output to an older sensor and this small difference can prevent a code being set FOR A SHORT PERIOD OF TIME. as the new sensor gets towards "midde age" the original fault will reappear. Something else in the fueling system is out of range.

Chassis221 post makes a good point regarding wire harness and should be eliminated before going any further.

If the wire harness is OK then I begin to suspect the MAF. However since the codes you provide are not OBDII definitions, with which I am more conversant, I can't be definitive.

If this were my job I would access the output from the O2 sensor to see what is going on. Is the O2 sensor voltage within range. Is it cycling correctly. If so the O2 sensor is OK and the likely cause is upstream of the O2 sensor which is basically reporting correctly but showing that the mixture/fueling is incorrect.

Since the O2 sensor has changed we need to be 100% sure that the correct type was fitted otherwise we potentially have two faults.

To proceed correctly we need to go back to the beginning and ensure that the vehicle servicing is correctly up to date. filters, plugs etc

Stuart

 

secondary air injection? last time i meet a problem like urs... the engine warning light appear... i try to clear the fault... then the car is ok.. after few ay... the customer complain the engine warning light apper again... the fault code something like urs.. after i check... the relay for the secondary air injector was wrongly plug to the diesel type engine place...

 

I'm new to this site and don't know if I'm doing this correctly but what I need help with is my 95 e 320. I thought that it was the air bag relay so I replaced It but Itt still will not start. Any suggestions??