How do I check the sensor for proper working order,first it is a 1990 420sel the car began to sputter and failed to take the fuel I unplugged the 02 sensor and it ran fine for 20 miles then started to act up again, did not help when I replugged the sensor. I need to know what the proper numerical value is when testing the o2. the other problem was that when I took readings at the 2&3 pins it would read 50.3 and steadily drop to 0,rev. the eng. and it jumps to 50.3 then back to 0(slow steady drop.)

The O2 sensor voltage reference value is .45 volts. Lower is lean, higher is rich.

Did this cycling start after you messed with the mixture adjustment?

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Ctaylor no I had not until after I replaced the ox sensor, I drove it 50 miles today and at 51 miles it started to buck and lose power again. I let it sit at the doctors office for 2 hours, and had no trouble starting, but I only got 15 miles and again it loss power, and would not take the fuel as you press the peddle, I stopped and put gas in and it drove almost home when it began to act up again. It seems strange that you can let it sit for half an hour or more and it starts and runs just fine, any how I want to go out and check the EHA setting and the potentiometer setting, AS I remember the eha was -0 to +0 but the pot I cant remember.

This sounds to me like a fuel supply, rather than a mixture, problem. Like the tank screen is clogged. It runs out of gas, sits, and enough dribbles into the filter and accumulator to keep it going for a while.

But it's so nasty to check that a look at the EHA current would be a good first step.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Yes I too would like to start at the top, My wife and I got our heads together, she reminded me that we had fueled in Shreveport at the same Chevron and had sputtering trouble on our way home and again this past week we bought fuel at that same place, But when we use the Chevron at home we have no issue with the car. I guess I may as well get with the program and empty the tank to be sure. I have replaced the strainer in the tank just last year and one fuel pump, it was a little loud. well back to the shop! thanks JNT

Come on, who are you going to listen to ?
Your wife, or us Einstein's here ?

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2007 C 230 Sport.

My wife for sure,, she can reach me!

up date it must have been bad fuel we have run two tanks threw and have not had a return of the bucking or the shut down trouble,though now we do have trouble with it falling below 500 on deceleration and not catching it self and then stalling.

Latest up date on this 420 sel and its poor performance on occasion's, low and behold as stated by Ctaylor738, It was indeed a fuel supply issue! Not knowing at the time of this original posting, that there was a filter in the fuel distribution unit, and that this was the culprit of our loss of fuel supply and the reason for its bucking and loss of power. this was found after my son who has had this car for quite some time and had this same trouble several times until he finally told me about it. I was looking for the answer again, when I had sent the fuel distributor off of my 190E 2.6 out for repair, I found the filter, I then pulled the line from our 420 and removed the filter from it and the trouble has gone away. I hope this helps others who have had similar troubles. JNT.