laEstrella

Hi,
What are the symptoms of bad oxygen sensor?
I've got a w168 A140 petrol from 2000 and trying to figure out why it is losing power. Upon inspection I've seen that there are two oxygen sensors in the exhaust but only one is connected and the other has no wires at all.
Where should that be connected? Can anyone point me to some video or resource where I can check it out?
What are the symptoms of a disconnected oxygen sensor? Could this be the cause the the car goes into limp mode at high speed? (if limp mode exist in this model at all)

Thanks!

Acta_Non_Verba

Oxygen sensors are placed in the exhaust before and after the primary catalytic converter. When you say one is connected and the other is not - think of "Front" or "Rear" in terms of describing them. Given that, now we can look at their functions and importance. The front O2 sensor is a primary sensor input back to the ECU and guides the ECU in determining fueling for the next combustion cycle. The rear O2 sensor is typically setup to verify the catalytic converter is doing it's job. When they fail, and they can fail in different ways, the front O2 sensors have far more impact on an engine's running condition and, yes, sometimes, based on the failure, can result in limp mode.

O2 sensors are measuring the oxygen level left in the exhaust after the most recent power/exhaust stroke. The reciprocal of the oxygen level is unburned fuel left in the exhaust. It is the unburned fuel the O2 sensor's job is to manage. When unburned fuel is high (Measured oxygen low), the O2 sensor signals a rich condition so the ECU leans out fueling on the next cycle, then when the sensor signals lean, the ECU richens the fueling mix.

A correctly functioning front O2 sensor produces a "wave" pattern - lean, rich, lean, rich, switching high, low, high, low. If you data logged a (front) O2 sensor output, it would look a little like a sine wave. Here is a DL I did on a car I was tuning. In this case, a dual exhaust V8 engine, has two front sensors (left and right banks), so two complete waves. Look at the fourth row of dark screen data. The correct technical way to describe each sensor is O2 B1S1 and O2 B2S1. These descriptors distinguish which bank, and which sensor (sensor 1 is front sensor; sensor 2 or even 3 will represent senor(s) past the cat(s)).


How do sensors fail? A couple of different ways, but I'll only focus on the primary way which is more critical to engine performance when operating under normal engine op temps. O2 sensors can hard fail or get weak. If they get weak (technically we call it getting lazy), either it no longer is capable of producing the high voltage, or possibly the lower voltage. As it relates to engine performance, if it is failing to signal the correct lean level then the ECU is only receiving rich signals so it is more consistently leaning out the fueling. This has an effect of distorting long term fuel trims (on the bank with the lazy O2 sensor) until it reaches design limits which is typically 25% (on any engine I've ever seen). Once at 25% lean, a CEL will be set. either a P0171 or P0174 (P0171 - bank 1; P0174 - bank 2). On the other hand, the opposite can be true, and the O2 sensor fails the other direction commanding the ECU to continually increase fuel and will eventually produce either a P0172 (B1); or a P0175 (B2). These are the conditions that likely might result in a limp mode, the lean condition being dangerous to engine health, the rich condition is poor running but not necessarily devastating. AND, BTW, given your MY, EMS design, you might NOT have a limp home option built in.

One last comment - the above is VERY basic, and there are numerous other possibilities (other than a failed O2 front sensor) that is contributing to a fueling problem, such as the MAF, which is the "front end" sensor to the fueling process, along with the MAP and the TB position sensor.

The O2 sensor you're describing that is disconnected should be the rear (or one of the rear sensors), and NOT a front sensor. If it is disconnected and not producing any CEL is is possible it have been tuned out. I've routinely tuned out rear sensor on cars where headers were installed or where cats have been eliminated.

Sorry for the length of this; hopefully it will help.

laEstrella

Thanks for the detailed explanation!
It is indeed the rear sensor that is disconnected and the CEL is not on. I'm not sure about headers but it still has a cat.
Actually my real problem is that at the speed of 70 - 80 mi/h (110-120 km/h). The car just suddenly looses power, it's like one cylinder stops working or goes into limp mode. At first it only appeared when traveling at this speed and I let off the gas pedal to apply the engine brake. Once I pressed on the gas again it didn't pick up the pace. My initial thought was that the spark plugs could be at fault but once I stopped the car and restarted it everything got back to normal. In fact when this happens on the motorway I only need to stop the engine and restarting it without stopping the car and everything goes back to normal until I got to that speed again. The car runs fine otherwise, mainly it is used for short shopping trips so this problem never occurs there.

I brought it to a non specialist garage and they made me change the fuel pump and the battery but none of those solved the problems. They were due to be changed anyway but those weren't the problem apparently.

So I had a closer look at it and I saw the missing O2 sensor cable I hoped that I may have found the problem. But if you say it can be tuned out then it must be something else like the maf or knock sensor.

laEstrella

On second though about the CEL. I unplugged the front O2 sensor too and the CEL didn't come on then either, nor I had any problem with running the car other than the problem I already have. So it ran just like always without any O2 sensor or CEL sign.

Acta_Non_Verba

So, interestingly, as I was reading your first reply, and before I arrived there, I was already thinking fuel pump.

I don't see this an an O2 sensor problem, but it is concerning if you ran the car with the O2 sensor unplugged and it did not produce a CEL. Something is up with that.

If you have a scan tool and are able to delete codes, try a quick test -
unplug the MAF.
Start the car
You should get an immediate or pretty quick check engine light
Shut the engine off.
Ign on, scan it and see if you get a P0101.
Then delete the code, plug the MAF back in start the car
The CEL should be gone.

If you don't get a cel on the unplugged MAF then something else is going on likely with the ECU.

The condition you're describing sounds like a fuel starved, or an air starved situation at moderate to high RPMs.

The list is a little long, but off the top of my head:
Fuel Pump
MAF
MAP
Really dirty air cleaner, or an obstruction somewhere in the air intake
Maybe I'm not ruling out an O2 sensor, but it would be based on how the EMS is constructed to respond to a failed O2 sensor. On older cars, the ECU might just revert to a fixed fueling table (which they all do), but not report a failed O2 sensor? On any late model EMS and manufacturer's design protocol would have a CEL set the minute the sensor fell out of expected range.

This is very difficult online, w/o seeing the car in front of me. But I'm giving you best guesses now.

laEstrella

Unfortunately I don't have any scan tool. I'm on the market for one but it's a tad bit confusing for the a140. I can connect to it any odbII scanner but not all are able to read out the codes, or at least this is what I could gather from forum post.

A mechanic also recommended changing the fuel pump again but as it was changed somewhat recently it didn't sound a valid solution. The air filter is newish and the a140 has the maf and ecu built together before the intake. If I disconnect it then I doubt the car does anything. I can try though.

Acta_Non_Verba

I wouldn't want to advise you further and discourage the MAF test I suggested if you do not have a scan tool as if you disconnect the MAF and cannot extinguish a CEL then you might find the vehicle running terribly without a quick remedy.

Suggesting, at this point, trying a different repair place. Sometimes a different set of eyes finds something the first place missed.

All the best with getting it sorted.

laEstrella

Turned out to be the fuel pump eventually.
I took it to a specialist garage where they found out that the ecu was remapped so it didn't show some faults. The ECU was set back to its original state then the necessary parts were replaced but the issue still didn't disappear. Luckily there was a problem with the fuel level indicator so the fuel tank needed to be dismounted again and there we found the culprit of the problem. The initially replaced fuel pump (that is also a Volkswagen fuel pump) isn't exactly like the original Mercedes fuel pump. It turned out that it could pull up dirt from the fuel tank and block the flow of the fuel. When the engine was shut off the fuel pump also released the dirt from the filter so next time the car started everything was back to normal until enough dirt accumulated again on the filter and blocked the flow.

Thanks for all the help!

Acta_Non_Verba

Very glad to see your car healed.

Too many like to "tune" these ECUs, then sell the car that way. I believe that is an unfortunate and wrong thing to do to the next owner. I am a tuner, but always restore any car I own back to factory specifications and factory ECU data tables before I sell. Based on the devices and SW used, some of the companies offering tunes in the past had locked the tune in making it irreversible. Glad you were able to restore the factory data.

Best,