Story of the check engine light with 700 miles or "How I fixed my own car".
 

As I posted in my initial 1,200 mile review of the E-class my new E-wagon with only 700 miles on the odometer and 10 days of ownership had to go back to the dealer due to a check engine light coming on.

The story starts with me picking up the car on 11/30 and enjoying it immensely for the next 5 days when all of a sudden weather turned horrible and "Icemageddon 2013" hit the DFW metroplex. We were snowed in all weekend and it gave me ample time to play with the car which included installing a black box dash cam system and an auxiliary 12V plug and USB charger under the glove box.

While I installed the dash cam I removed the cover from the built in stereo camera and I also disconnected said camera to better route the additional wiring I pulled in for the new black box. I also made the mistake of turning on the vehicle without the stereo camera being plugged in which ended in copious amounts of warning messages about Active Lane Assist being inoperable and Distronic+ not working.

I immediately knew what I forgot to plug back in, I shut down the car, plugged in the camera wire and assumed everything would be back to normal. Well, things weren't as I got a "check engine" light in the dash with no information attached to it other than the yellow light being on.

I was hoping the light would go away and the error would clear as I drove around as I eliminated the open circuit. But I was also skeptical about me unplugging the camera having anything to do with the check engine light as the light is strictly reserved for emission problems and is a government mandated requirement in every vehicle just as Airbags and TPMS sensors.

Either way, I took the car to the dealer on Tuesday where I told them what I did and asked them to clear the error code which was probably due to me messing with the car. They cleared the error code, gave me the car back after an hour and I was on my way. The light was off and everything seemed fine until later in the evening when I went to pick up my wife from her workplace, it was still too icy and cold for her to be comfortable driving. As soon as I picked her up the check engine light came back on.

So I drove the car to Autozone and had them pull the codes, as at this point I was certain that the check engine light had nothing to do with Distronic+ and probably something to do with engine emission and management. The car had two codes: P0071 and P0111. I did not ask them to clear the codes but once I went home I learned that P0071 had to do with Ambient Air Temperature Sensor Performance problems while the P0111 code was indicating the same problem for the Intake Air Temperature sensor. I also ordered a Bluetooth ODB2 Monitor from Amazon.

So I took the car back to the dealer on Wednesday and told them that I found these error codes and the check engine light was back on. I also told them that while I did install some auxiliary equipment in the car it should not have had anything to do with these problems. They gave me a loaner car and sent me on my way.

The service manager also called me and the tech who was working on the car asked what exactly I connected where when I did my install. I felt a bit like they were thinking I was just another idiot who randomly connected some wires under the dash until things worked. So I jumped in the loaner and drove to the dealer and showed the PDF copy of the wiring diagram of the entire car that I got from WIS (Workshop Information System, the official MB repair manual) which showed that I did my install right and I knew what I was talking about. But by then they already took the inside of the car apart to double check my work which they found no problems with.

They checked the car and said that usually this happens when the ambient temperature senor gets dislodged from the bumper and fails. But mine was intact and in place. So they ran some more checks and diagnostics and they called me up on Monday that the code is cleared, they replaced the Ambient Air Temperature and the car is good to go.

In the meantime my ODB2 Monitor arrived and I used it on the loaner car for fun, no error codes were generated but I monitored its Ambient Air Temperature and Intake Air Temperature over time. When the car was cold the two sensors read very similar values and once the car warmed up the intake temperature was 10-15F higher than the ambient one due to residual heat from the engine. I found it also interesting that the temperature reported on the dash was 1-2 degrees higher than the one reported by the ODB2 data.

I picked up my car Tuesday morning in freezing temperatures, the repair ticket indicated they test drove the car twice for 20 miles total and the check engine light never came back on. I drove the car from cold start 4 miles and the ODB2 monitor already registered a pending fault code of P0071 and within another 4 miles I got the check engine light back on and a Confirmed Fault of P0071 which would indicate Ambient Temperature Sensor fault.

I was also monitoring the Ambient and Intake temperatures and while ambient read right around 30 F the intake was reading 116 F. Given that it wasn't June in TX I knew which of these readings were wrong and it wasn't the ambient temperature.

So to me it was obvious that the system compares the ambient and intake temperatures and if they differ by a certain amount the engine throws an error code. When the techs were testing the car in the afternoon in 60F weather the difference was small to stay under the threshold but when I drove it in the cold morning the difference went over the threshold and it threw the code. And this was also the reason why the check engine light started happening when the cold snap hit my town which just happened to coincide with the installation I performed.

So I took the car back, showed them the temperature logs from my ODB2 scanner on my cellphone and told them that it's the Intake Air Temperature sensor that is reading wrong and left in the same loaner I had before. They didn't even touch my car that day saying they wanted to reproduce the problem themselves next morning when the car was cold again. Wednesday at noon the service manager left a message saying that they looked at the IAT sensor and since they didn't even have that part in stock they pulled one from a new car on the lot, swapped it out and it was still reading the wrong temperature and they were down to checking connectors and wires and they were still wondering if there was ANYTHING I might have done while did my install. My wife told me "just go and get your car back already, they can't fix it!" I told her "There is still a check engine light, I'm not going to pick it up unrepaired". I called them back and assured them that based on the wiring diagram the IAT sensor is directly connected to the Engine Control Unit and it's a wiring harness that does not even leave the engine compartment, it goes from the intake side of the engine to the ECU side and that is all it does and it has zero effect on what I tied into under the dash.

I did drive by the dealer Wednesday evening and while my service manager was gone for the day, the tech who worked on my car was still there. He said he pulled the sensor, checked the car with another sensor, checked the wiring and they all checked out so he was planning on ordering a new ECU the next day. A $1,200 ECU!!! I went home but as I was driving home I remembered that on this particular engine there are two temperature sensors on the intake side of the engine, one is called Intake Air Temperature Sensor and the other one is called Intake Manifold Intake Air Temperature Sensor. They're identical sensors but one is on one side and the other is on the other side of the engine under the engine cover. So I fired off an e-mail to my service manager at midnight telling him my concern whether the tech checked both of these or just one of them. I even attached the corresponding page with location markings from the WIS to the e-mail. He forwarded my e-mai to the tech and by 9AM next morning I had a call from him saying "Guess what? You were right, it was the other sensor". It turned out the tech swapped the Intake Manifold Intake Air Temperature Sensor and not the actual Intake Air Temperature Sensor first time around.

So after 19 days of ownership with 8 days in the shop I finally got my car back. The lesson to be had is that mechanics don't work on a car until it's fixed, they do a little bit of work then go change oil and rotate tires to make some money and leave yours in hopes if some miracle or divine intervention fixing it overnight. They're also highly skeptical of anyone installing anything in their cars even after showing them proof that you know what you're talking about. They also often check the wrong part and can't follow the instructions. They also lose their pants on problems like this as they spend so many hours on finding out what is wrong while they probably can't bill the entire time under warranty.

Bottom line is that it was a simple fix that should have taken them an hour tops while it ended up as a week long saga and me in a E350 loaner for 550 miles. I'm going to have to give the dealer a review about my service experience and while they were as nice and accommodating as possible they were equally incompetent from my perspective. Was it not for me they'd be installing a new ECU today only to find that everything is still wrong and not working.

Holy smokes. Great review and great insight. Which Park Place dealer was this? Guess you don't need a MB technician these days to get a job done correctly...
Glad you didn't have to get a new ECU.

Qtip, it was the Fort Worth, TX Location. I worked with the Assistant Service Manager, Nick DeLuca and like I wrote he's a super nice guy, they put me in a nice E350 loaner with Distronic and didn't blink an eye when I returned it with over 500 extra miles on the odo. The tech was Dustin, another nice gentleman.

I only have my side and my perspective on the story, I had way more time to look at wiring diagrams and sensor locations at night at my leisure than they do while on the clock trying to bang out service requests but somehow I still feel that their diagnostic skills need improvement.

I really don't know how to rate the service when you know that anything less than a 10 is a fail for them.

It takes an engineer to find this fix.

The problem is that the diagnostics has not kept up with the smarts of the car. It should have logged readings and provided more information after a fault. That would have provided the whole picture for the tech to understand an issue better.

Thanks for the post.

Finding a good doctor, mechanic, engineer, any good professional is always challenge. So help yourself is very actual nowadays.
For example my daughter had allergy and it was obvious even for me however a highly reputable doctor prescribes her antibiotics, it is hilarious.
I guess my bad luck with MB got over, no any bad lights homing up for last 6 years.

To me the bottom line is that you shouldn't be screwing with a brand new, under warranty car, plugging and unplugging modules and adding electronics.

But YMMV.

Uhm... why not? Because other people's incompetence might try to blame you for something?

It wasn't my first rodeo with installing something as simple as an AUX 12V connector or splicing into two wires for 12V and ground. I did a bit far more extensive work in my younger days: http://www.audiworld.com/tech/elec82-1.shtml

I even made home made special antenna adapters for VWs for aftermarket radios before anyone on the market has done it: http://forums.triplezoom.com/showthr...non-VW-H.U.%29

Again, I'm just saying I probably knew more about the electronics about the car than the tech who worked on it and I felt a bit insulted when they questioned my work...

temperature sensor shouldn't have gone bad with electronics disconnects. Aren't they just resistors? I would assume they only degrade due to heat or surface contamination.

I agree. The techs still use the diagnostics system that is stationary and sitting in the shop but they don't have the capability to log live data while on the go. I logged more data with my car than they ever did while in the shop. When you can buy an ODB2 Bluetooth scanner for $20 and hook it to your phone the factory tech's capabilities seem rather limited. But I still find it hard to excuse that he was checking on the wrong sensor to begin with especially after I showed them the logged data.

I feel the same way about doctors as I feel about car mechanics except I can't perform surgery on myself and if someone screws up the mistakes can be far more costly. This is why doctors carry malpractice insurance while car mechanics don't.

Indeed. And my guess is the sensor was faulty from day one but the problem didn't surface until the weather got cold enough where the temperature differential between the ambient and the intake air temperature sensors has gone over the threshold.

Agreed. Once you start tinkering with 'a brand new, under warranty car, plugging and unplugging modules and adding electronics' there is automatically going to be some suspicion on the part of the dealer/mechanic as to your skill level and what exactly you did or claimed to have not done.

Also for the skill level claimed, for example as a pro electronics shop will also need to if they mod a brand new car, you may have to contribute additional expertise of your own in collaboration with the dealer/mechanic to solve the problem. This would be expected and goes beyond the replacing of oil and tires.

dude
you have a big problem
you are too smart for MB owner , you should own a BMW not a MB :naughty:

The electronics I "added" was no different than someone plugging in a phone charger in the cigarette lighter. It was exactly the same load and if you had to map the wiring it would look identical to that scenario on difference being that I "plugged it in" behind the lighter socket.


Why should I or anyone be expected to collaborate with anyone? When I go to the doctor they don't ask me "well what do *you* think it is, sir?". They actually hate "Dr. Google" who self diagnose.

The temp sensor would have failed whether I installed an additional power outlet or not. The tech would have missed it the same way. I could have pulled all that I added to the car and put it back to stock and played dumb. I'd still be driving the loaner today. The only reason I contributed was because I wanted my car fixed and I wanted the least amount of tearing apart performed by the tech on my car. The more they touch the higher the chance of them breaking it, not putting it back the same way or leaving smudge marks on my car.

If the dealer is only good for oil changes and tire rotations I might as well stop going there. The advantage they have is that they have the right tools, the right equipment and the right parts. I also hoped that they also have the right expertise.

It's obvious who's an engineer and who isn't....


I traded my BMW E90 for this E-class. If BMW made a mid-size station wagon I might have stuck with the brand but it's hard to beat the utility of the E-wagon.

I see
your behave like a BMW owner indeed
GL !

I don't even know what that means...

Great write ups Greg. You are very detail oriented about your experiences. Keep them coming!

"While I installed the dash cam I removed the cover from the built in stereo camera and I also disconnected said camera to better route the additional wiring I pulled in for the new black box. I also made the mistake of turning on the vehicle without the stereo camera being plugged in which ended in copious amounts of warning messages about Active Lane Assist being inoperable and Distronic+ not working."

Where did you end up mounting the camera? Are you satisfied with it the location? Do you have a pic of it mounted? I am interested in picking up one but unsure where to mount it due to the space the lane keep and night view cameras consume above my rear view mirror.

Obviously "dash" is the option but I would prefer somewhere else more permanent and discreet.

I am curious about the dashcam too. You should document that install - I'd love to have one in my car to document the skills of the local motoring population, but I can't stand having loose wires in view.

I'll post up a write-up about the install although since I did it in the middle of freezing cold temperatures I was too lazy to take photos while I did it which means there won't be photos of the car being taken apart. It was a bit more involved than a simple dash cam install as I went with a dual camera setup and I had to route a USB cable that carries the video signal and power to and from the rear camera which is mounted on the back window of the trunk door which meant fishing the wire through the entire length of the car and getting it through into the rear door.

The dash cam is attached just under the huge compartment that contains the stereo cameras and the rain sensor and it's behind the rearview mirror so it's hardly visible from the driver's seat. There was also a GPS module that I first hid inside the compartment but I had GPS reception issues in overcast weather so I moved it right next to the big compatment on the passenger side. The only thing I haven't found a good place for is my TollTag.

Tonight I did my first "hack" that would be potentially hazardous to the dealer's equipment as I added an extra pin and connector with switched 12V to the diagnostic socket so I can keep my ODB2 monitor plugged in at all times without it draining my battery. So I would have to disable this connection before I'd take the car for service as the Mercedes specs define N/C (no connection) on this pin and I could be blamed for frying their diagnostic unit with 12V although I doubt that would be the case as other manufacturer's use pin 1, for example VW/Audi put a switched 12V on this very port already hence my choice to use the same pin.

The switched 12V drives a Fujitsu Automotive quality relay that I installed inside the ODB2 diagnostic box and it switches the constant 12V on or off for the unit so the power drain is off of the ODB2 connector not from the switched 12V I only use the switched 12V to drive the relay solenoid.

It was certainly sarcasm because the poster is pretty sure that BMW creates much more problems than MB. Perhaps it is true, and it is why MB dealers do not spend big money to pay highly skilled technicians. They are fine with average mechanics capable to do oil change or tire rotation. It is certainly compliment to a quality of MB cars.

:word: Good logic with your diagnostic efforts. The expectation that repair people should know everything about what they are working on is not valid for the simple reason that they are not engineers but parts replacers.....which makes you wonder why they would question someone adding to or modifying their own car. Most learn from experience, not training and we couldn't afford to pay people with engineering degrees in all the systems in a car to repair them. I bet this shop won't burn themselves on this failure again and unfortunately you happen to be the guinea pig.

In defense of the "parts changers", quite often they are better at solving problems just because they have seen particular problems before.
They're in the trenches day in, day out.
Not all engineers make good techs. While we certainly understand the theory and we can make good finds like Greg did but quite often we can get hung up on unrelated minutia.
It's also because the don't have the theoretical background that they will question somewhat unrelated changes.

I didn't mean to sound too harsh or negative about the experience because it wasn't other than the egging I had to endure at work from my co-workers for "MacGyver-ing" my car and having a new car in the shop for over a week because "I screwed something up" and the sleepless nights and doubting myself about whether I did or not.

The problem in this case was the lack of proper diagnostic skills and it was more like "Let's change this part and see if it fixes it".

The error codes that the car originally threw indicated a misread between the ambient air temp sensor and the intake air temp sensor. So it would have been a natural progression to try to decide which of these two sensors are reading wrong. Based on the readings from a cold morning it would have been obvious that it wasn't the ambient sensor but the intake one.

The second step would have been to check the sensor itself and not by simply swapping it out and see what the new one reads but by hooking an ohmmeter to it and comparing the resistance reading to the nominal reading per the service manual, it's all in there.

Then the third step would have been to make sure you're checking the right sensor and not a different one.

I'm positive that the tech who worked on my car will not make the same mistake again. They also started with the obvious which would have been the faulty ambient air temperature sensor as they have seen that one fail before on other cars.

I'm not really holding this one against them, I understand where they're coming from and what they have to work with and how complicated these system really can be and often are. I never got irritated or frustrated with the service manager or the tech at any point in the process, there is no point to get worked up over it as it accomplishes nothing. And this is precisely why I'd hate to give them a bad rating if it ever comes to a questionnaire.

Agree, and that was one of my points. Nothing wrong with experience over training or schooling except there is always the first time failure that <someone> is the victim. Also, there is nothing derogatory about being a parts changer with today's technology.

Yes. Point being that technicians don't have the knowledge level of engineers nor should we expect them to. Repairing anything that is broken is not a science.

I can also understand that every time someone comes in with anything added to the car by a 3rd party they will be a bit skeptical. The service manager said "You wouldn't believe some of the stuff that gets put in by Cartoys or some owners". The only thing I can hope for is that my file got a footnote that says "this guy knows what he's talking about" and not "This guy is a know-it-all".

Greg great post. Thanks for sharing. I'm sure the experience will make Dustin a better tech.