1994 C280 Oxygen Sensor

[BillKerler]

My oxygen sensor AFTER the catalytic converter had its heater element fail (check engine light code PO141). The car put me into limp mode about 15 minutes later as I was trying to get home (CEL code PO300). The engine was running very rich on the air/fuel mixture (over 5% CO).
I ordered a new oxygen sensor and proceeded to replace the spark plugs after running on the misfiring limp mode.
I reset the engine codes with my Acton readout device for PO141 and PO300.
My new oxygen sensor arrived and I put it in a vice and hooked up my positive lead of a digital voltmeter to pin 4 and the negative lead to pin 3 and then ran a propane torch flame over the tip of the sensor. Within 20 seconds I had a half volt readout. When I took the flame away, within 4 seconds the voltage dropped. Okay, so the new sensor seems to work.
I installed the sensor and plugged it in. When I start the car, it runs better than it did in the limphome mode, but's it's not right. The CO reading on my home meter indicates 1.7% which is still way high (anything over 1% is high).
Question, do I have to RESET the emissions control/fuel injection computer?? Since the heater element is what broke on the original sensor, is there something to check to make sure the car computer is sending the right 12 volt signal to the heater element for the new oxygen sensor? Do I just keep driving and after 10 start/stops will the computer gradually correct itself? Thanks

[Cru328prod]

Did you buy a boshe replacement? Our benz's are reallllly picky little whor3$....like if you put a ebay cheapo MAF you'll get CELs

[BillKerler]

I actually bought a Bosch oxygen sensor from an online parts store that shipped from back east on FedEx ground. By the time I received it, it did not work...there was no output from pins 3&4 with the propane flame put on it. These sensors easily break if dropped and I think shipment did it in.
So I bought a sensor locally and Denso was supplied. The Denso put out a nice strong electrical signal with the propane torch test.
The strong signal should be the important factor I would think.

[Sulaco]

This really makes me wonder....

My O2 sensors (1994 C280, 235k miles) are factory original.

[BillKerler]

You bought your car "used" on 5/3/99...5 years of previous owner driving...was it at 90K to 100K on the odometer when you bought it? It may have been replaced just before you got it. These sensors are suppose to be good for about 100K miles but certainly some can exceed that. 235K is way more than expected life. As a note, mine broke in the heater element portion as I took my RPM's above 4500 and that pretty quickly cooked it. I figured it was on its last legs (I have 192K miles and this sensor was last changed at about 95K).

[Sulaco]

Hi Bill. Long time!

It had 14k when I bought it. It was a company car that was in a minor rear-end accident, so they sold it as a write-off after the repairs. After the repairs it sat in a garage for 4 years before I bought it. I got it for a DEAL. It still smelled new...

So, almost certainly the original O2 sensors.

[xvvvz]

Bill - Were you able to get that second/downstream O2 sensor off without dropping the exhaust? I have this repair coming up soon and any tips would be appreciated.

[BillKerler]

The exhaust does NOT have to be lowered if you have the oxygen sensor removal socket. I bought mine for $4 at Harbor Freight tool company. I believe it is called a crow's foot socket. It has a slit along the socket length to pass the sensor wire lead through. Different from a regular socket, where you plug in the 1/4" male end is off to the side at a slight angle on a crow's foot.
I used quantity 2 universal swivel joints in series plugged into the socket 1/4" female connection...you plug into the back side of the 1/4" crow's foot opening. Then I used 1 long extension piece and plugged it into the swivels plus the rachet at the other end of the extension piece.
Two things...1st, connect the swivels and extension into the crow's foot oxygen sensor socket and then work the socket into position on the sensor itself...I had the car jacked up on stands and had plenty of room to work with both hands. Don't try to plug in the swivels/extension after the socket is in place on the sensor, you will go batty trying to do it.
2nd, even though you want to remove the sensor and it has normal threads, twist the rachet in a clockwise direction...ass backwards due to the geometry of plugging into the back side of the crow's foot socket.
My sensor popped right out...no cussing or head banging. If yours gets stuck, Mercedes recommends running the car for about 5 minutes to get the exhaust pipe up to 100F hot and that is an easier temp to break a frozen (ie, rusted) connection. 100F will not burn your arms and hands.
It's very straight forward.

[xvvvz]

Thanks, Bill. That was exactly the insight I was needing.