1998 Mercedes Benz ML320 check engine light

The oil change has nothing to do with it for the oxygen sensor code to pop-up-If the CEL comes on again-Scan the computer for codes-

Excuse me for butting into your conversation but I'm compelled to add a few comments. First of all, your oil wasn't over-filled. To cause it to spill over would take a good 30 quarts, probably more, to completely fill the engine. Anyone with an ounce of common sense would recognize they were doing something wrong long before they put that much in. What most likely DID happen was the guy just plain spilled. We all do that from time to time. Typically we just wipe off what we can reach and let the oil burn off that runs into tiny inaccessible places.

The next thing is you have to look at the diagnostic fault code rasmataz wants you to have read. Even though the Check Engine light turned off, those codes will stay in memory so they can be read. You must be aware though, fault codes never say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis. It is true that oxygen sensors use outside air for comparison to the amount of unburned oxygen in the exhaust. The outside air can be blocked by an exuberant rust-proofing technician who coats the sensor with that material. That, and oil, do not destroy the sensor. They simply prevent it from doing its job. The proper repair is to wash it off.

That's where the fault codes come in. There's about a dozen different fault codes related to an oxygen sensor and they point to different failures with different fixes. About 3/4 of the codes refer to an unacceptable condition being reported by a properly-working sensor. Of the rest of the codes, probably half are the result of a wiring problem related to the sensor. An oxygen sensor will only solve a fault code less than half of the time there IS a code.

Also, when you DO need to replace an oxygen sensor, you don't replace all of them. There is no magic time or mileage at which they begin to fail, and they don't wear out at the same rate like tires or spark plugs do.

Finally, you can get an idea of the seriousness of a fault code by how the Check Engine light acts. There's well over 1000 potential fault codes, and about half of them do not even turn the light on. Codes that refer to something that could adversely affect emissions must turn the light on. If that problem is relatively minor, the light will turn off while you're driving when the problem goes away. If the problem is a little more serious but it goes away, the light will stay "latched" on until you turn off the ignition switch and restart the engine later. If it's still more serious, the light will turn on anytime the engine is running, even if the problem stopped acting up a while ago. For all of those conditions you can keep on driving, often with no noticeable drop in engine performance, but the codes should be read as soon as possible. Many minor and inexpensive problems could turn into expensive ones if the cause is ignored.

It's when the Check Engine light is flashing that you must "bail out and run for your life". Actually, it means stop the engine right away to prevent damage to the expensive catalytic converter(s). Too much raw fuel is going into the exhaust system and the catalytic converter is going to overheat.

The other clue that this is not related to the spilled oil is the problem did not occur right away. All engines have oil leaks when they get to be a few years old, and if oil spilled three days ago would cause a problem, cars with constant, very small leaks would all be developing the same problem. In reality, any leaking oil that headed toward an oxygen sensor would vaporize and burn off long before it reached it. Oxygen sensors don't even start to work until they reach 600 degrees (F). Oil in the engine will barely reach 250 degrees, and at that temperature it is already starting to vaporize. If your mechanic DID manage to spill enough oil onto a cold sensor, it would have burned off in a very short amount of time. That oil would not wait three days to cause a problem.

What I would do is record specifically what the fault code said, then erase it and see if it comes back. Either get the exact code number or its description. The codes get very specific and each one has its own troubleshooting steps. If your mechanic just says it's an oxygen sensor code, that's no more helpful than saying "tire" or "wiper". You have no idea if one has a problem, all have the same problem, something is worn out, etc.

There are incompetent people in every profession, and there are really good people who make mistakes from time to time. Doctors just bury their mistakes. Ours keep coming back.

Your first mistake is thinking of mechanics as your adversaries. As with teachers and students, think of mechanics as your advocate. They want what's best for your car and for you, but they usually do a very poor job of communicating those concerns. That's the cause of a lot of mistrust.

I'm still not convinced the oxygen sensor problem is related to the oil issue. Any problems would have shown up right away, not days later. Did you have the oil changed at a speedy lube place? If so, they usually have two or three people working on the car to get it done faster. Sometimes one person puts the oil in not realizing someone else already did. That still wouldn't be enough to run out the top. That's why I think they just spilled. The typical fix for over-filling is just to drain a bunch back out, then refill as necessary. Usually no engine damage results because you will notice poor performance or the oil light will turn on and you'd go back to the shop long before serious damage occurred.

The engine actually can be damaged by too much oil. It's not that the oil is harmful. What happens is the level is so high that it gets hit by the spinning crankshaft and that whips the oil around and adds air to it. Air can compress. Oil under pressure is needed to isolate engine parts from pounding against each other. Air in the oil reduces its ability to do that. That results in accelerated wear, not instantaneous damage. No damage will occur over a short period of time.

Since the air can compress, the oil pump is less able to build pressure. THAT's why the oil light may turn on; not from being over-full.

Also be aware there are just as many guys out there who are clueless about how their cars work. Over nine years teaching Automotive in a community college, three of my top students were girls and the guys had a lot of respect for them. YOU may feel uninformed when you walk into a shop, but the person behind the counter knows there are a lot of sharp people out there who know how their cars work, and you could be one of them. The people behind the counter at the dealership I used to work for know I was a mechanic and a teacher, but they still describe stuff to me in a way that makes me roll my eyes. You're getting the same treatment and you're feeling the same way, not because you're female; it's because their job is to make something they don't know much about and to translate it from something they misunderstood from the mechanic into something they think you will understand. That is not an intent to defraud or deceive. It's why we include classes on oral and written communication in our Automotive programs.

The next time you talk with someone behind the counter, remember they may know less about your car than you do. Their job is to relay information from the mechanic, and mechanics talk a totally different language that the average person can't understand. That's when you come back here and let us interpret what you were told.

So, ... You fell for my snow job! I have a lot of people fooled.

Actually, I'm not very familiar with Mercedes products, but I can offer some suggestions. I'm very familiar with Chrysler products. They were the leader in innovations that actually benefited car owners since the 1950s. One of their developments includes the computer-controlled transmission in 1989. Admittedly it had its problems, all related to "tolerance build-up" on the assembly line once they started to mass produce them. Today every manufacturer uses their own version of Chrysler's design, so what I'm going to share pertains to my knowledge of Chrysler's transmissions. Use my wondrous knowledge to get an idea of how to proceed.

All automatic transmissions use fluid under pressure to apply the various clutch packs. The fiber and steel plates in each pack wear away with use. As that happens, it takes more and more fluid to apply the clutches. On older cars we would notice "engine runaway" when the transmission was showing its age. That's where the engine would speed up, then come back down between up-shifts from one gear to the next. It was due to the next higher clutch taking too long to lock up. Once it did lock up, you continued driving like normal. This could give you a year or two of warning that a rebuild was in it future.

With Chrysler's design, the Transmission Computer learned the volume, in cubic centimeters, it took to apply each of the four clutch packs. As wear takes place, more fluid volume is needed, and those values are updated in its memory during each up-shift. How fast that volume fills can be related to your driving style, and that is also learned. The result is when the computer knows it's going to take more fluid volume to get a clutch pack locked up, it starts filling that assembly sooner. To say that a different way, it might apply the clutch pack(s) needed for third gear, then wait just a little longer than normal, then release the clutch packs for second gear. Third gear will finally be locked up just as second gear is releasing late. The advantage to that is you always get the nice crisp, solid up-shift like it had when it was new. The disadvantage is you don't get that one or two years of warning that wear is taking place. One day it shifts fine; the next day, without warning, you have a problem.

The computer will continue to update the fluid volumes until the day comes when it can't update any more or when the clutch plates are worn so far that no amount of fluid will get it to lock up solidly. Then slippage occurs. Even though slippage is taking place, the plates have enough pressure yet to cause friction and serious heating of the fluid. On any automatic transmission that continued slipping will overheat and burn up other parts real fast and turn the entire transmission into junk.

That slipping is detected by the computer by comparing "input speed", which for this sad story can be considered to be engine crankshaft speed, to "output speed' which is the drive shaft or half shaft speed. It knows which gear it has commanded to be applied, so it knows what road speed must be for a given engine speed. When there is a discrepancy, it is because that slipping is taking place. To prevent further damage, the computer defaults the transmission to "limp mode". That gives you a choice of "park, "'reverse", "neutral", and for any forward gear it will stay in second gear. That allows you to drive slowly to a repair shop without needing a tow truck. Some people don't understand that and try to drive 70 miles per hour with the engine screaming, then they blame their engine problems on anyone available but themselves.

Along with limp mode, and a whole pile of other problems, the computer will detect them and set one of hundreds of diagnostic fault codes, (DTCs). It's important to have those codes read and recorded soon if it's an intermittent problem because they can self-erase after 50 engine starts if the problem hasn't reoccurred. You want to know what that code was for in case you have more trouble later. You must also not disconnect the battery or let it run dead because that will erase any codes, AND, ... It will reset the learning that took place in the Transmission Computer, as well as sensor data in the Engine Computer. All of that data will be relearned when you start driving again, but the transmission could shift real sloppy at first or like a race car. Mechanics are supposed to drive your car after doing anything that required disconnecting the battery, or they are supposed to explain to you the need to wait for the relearns to take place. Typically it takes less than a dozen up-shift cycles or two to three miles for the relearning to be complete.

As a side note, the first 100 of these transmissions were hand-assembled and measured, and those had no problems. One in Chicago was known to have well over 150,000 miles with no repairs needed. Their horrible reputation came from that tolerance build-up once they started coming off the assembly line. A sealing surface might be okay if it was off-center 1/16" one way or the seal could be off 1/16" the other way, but if you had both in the same transmission, internal leakage could occur resulting in a clutch pack not applying properly. Even worse, that seal might do its job today, but not after 50,000 miles of wear to other parts took place.

30 of these transmissions exploded right on the assembly line because they would lock up the gear they were shifting into before the one they were leaving was unlocked. There were two rotating masses spinning at different speeds, and both were locked together AND to the housing. To address that, overnight a design change was implemented in one of the integrated circuits, (computer chip), and was in the computers the next morning. Normally it takes six months to design a chip and get it into production.

In actual practice, you are likely to not even notice the transmission going through its relearn procedure after the battery was disconnected.

I have to add some information on the other diagnostic fault codes that can be set. Some relate to a sensor's electrical circuit, to include a break in a wire going to it, a corroded connector terminal or splice, or a defect within that sensor. The computer can only detect that something is wrong. It doesn't know what exactly. This is why fault codes only direct the mechanic to the circuit or system that needs further diagnosis. They never ever say to replace parts or that one is bad.

The computer also watches for the results of its actions. For example, when the torque converter locks up for better fuel mileage, (another Chrysler invention in the mid '70s), it knows engine speed had better drop about 200 rpm at highway speed. If it doesn't see that in response to it commanding that clutch to apply, it sets an appropriate fault code. Those codes refer to "unacceptable operating conditions". That term is mine!

Years ago all automatic transmissions were controlled hydraulically by over a dozen valves in the "valve body". Some valves moved in response to engine speed, some due to road speed, and some when pressurized fluid was switched on from one of those first two sets of valves. Some were spring-loaded to return to a certain position, some weren't, and some could stick due to varnish build-up or a very tiny piece of dirt getting caught in them. All of those valves ultimately directed fluid flow to apply one or more of the clutch packs.

With all of the computer-controlled transmissions, the valve body has been replaced by a solenoid-operated valve for each clutch pack. A solenoid is either on or off, (applied or released). There is no in between, and that would make for a very uncomfortable bang when shifting into drive or reverse, and a similar bang when down-shifting into first gear when coming to a stop. To address that, if you listen to any older Chrysler vehicle, you'll hear a light buzzing, or ratcheting sound when you shift into gear, and again when slowing to a stop. That is the sound of a solenoid being pulsed on and off dozens of times in about one second to modulate or soften the engagement of a clutch pack.

There was a list of about a dozen complaints with this transmission, all related to comfort, that were addressed with updated software to modify the "shift schedules". In the case of my '93 Dynasty, I didn't like the "bump shift" which occurred when it waited too long to down-shift to first gear when approaching a stop sign. Often it down-shifted just as I was starting to accelerate away, and I could feel that bump. By the mid to late '90s almost all other manufacturers had developed their own designs and they learned lessons from watching Chrysler's learning and development. Today those comfort issues no longer exist on any brand that I'm aware of.

The last point of interest has to do with that continual learning and updating of the shift schedules. You don't realize there's wear taking place, but a transmission specialist can find that out for you. I have a Chrysler DRB3 scanner for my personal vehicles but I'm pretty sure many aftermarket scanners will work for this. Under one of the menus you can read the "clutch volume index", (CVI). That is a set of four numbers correlating to the number of ccs of fluid it takes to apply each clutch pack. As I recall, 80 was the highest. If you found 75, for example, you'd know you were getting close to having slippage and the need for a rebuild. Use those numbers for illustrative purposes only. I'm not sure what the exact numbers are.

So now you know as much as I do. Your transmission is going to incorporate some parts of this story, and for the code you listed, there's about a 50 percent chance the sensor itself is bad. It's just as likely there is a wiring problem related to that sensor. Professionals will always test and inspect the wiring first before ordering a sensor that usually can't be returned. This code is referring to the circuit itself. There are totally different input speed sensor codes that refer to a mechanical problem that has been detected through a properly-working sensor. The first place I'd look is at the terminals in that sensor's connector. Look for corrosion, and since you don't know the history of the vehicle, look for "stretched" terminals. Those are the result of someone poking in there to take measurements and they spread one of the terminals to the point it doesn't make good contact with its mating terminal. Those can look okay but cause a pile of intermittent problems. One potential way to find those is to erase all the codes, and when they don't set again right away, wiggle on the connectors and harnesses while the ignition switch is on and / or the engine is running. Have a helper sit in the car and watch the Check Engine light. That light turns on when a fault code is set related to anything that could adversely affect emissions. A car stuck in second gear would burn more gas than normal, thus the increased emissions.

By the way, most people think their car is stuck in first gear when it's in limp mode, but it's really second gear. On other brands it might really be stuck in first gear. You can also get a clue to the cause of the code by when it goes to limp mode. Electrical circuitry problems, including defective sensors, are detected as soon as you turn on the ignition switch, and you'll be in limp mode as soon as you shift out of "park". Slippage in a clutch pack is detected when it occurs, and it will bang back to limp mode at that time. That occurs when you're already moving, and it could occur after you've been driving for minutes or hours.

If you don't find the solution, or a clear course of action in this novel, you should start a new question. Once I posted my original reply this became a conversation between just two of us and anyone researching the topic. None of the other experts will ever see your addition or have the chance to reply. That does you a disservice because you're stuck with just me. Be sure to include the engine size and the mileage. We often base our recommendations on the vehicle's mileage and / or age.