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Do-It-Yourself H.Q.
Share your DIY projects or ask questions about how to fix something on your own. Help fellow Bimmerfest members improve your wrench turning skills! All BMW DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 07-03-2013, 09:42 AM
therug79 therug79 is offline
Registered User
Location: Western Australia
Join Date: Jul 2013
Posts: 2
Mein Auto: BMW 120i E87 N46
Talking BMW 120i E87 N46 Engine Misfire and O2 Error Codes - From Lemon to Legend, our story!

Hi All,

I thought I would share our experience on this forum for two reasons:
1. This forum was a source of a lot of ideas for me to contemplate whilst having issues with my car. I wouldn't have gained the knowledge I have now without it, so it is only right I pass on my own experiences to help others.

2. Whilst the above codes were quite common and had frequent information about them, the actual cause was nowhere to be found when searching for those codes. Hopefully this post will pop up and help those who have the same problem.

We purchased our 2005 120i E87 N46 car late December last year and everything was fine on the first few days. Luckily, our dealership (although they knowingly sold us a lemon - as we found out later) includes a premium (3rd party) warranty with all their cars.
Over time some symptoms appeared and gradually got worse.

Firstly, no engine warning light was coming up, but there were obvious issues with the car. At low RPM/idle the engine would shake to the point where all passengers were being 'rocked' in their seats.
After a while, the engine light would come on then disappear after a while.
The symptoms were worst when the car had eventually entered 'limp home' mode (accellerator to the floor but not going faster than 50 km/h).

We eventually took the car to our dealership who had their own workshop look at it. They could not find the fault and said 'nothing wrong with the car'. By this time, the engine light was on permanently and I started to look into diagnosing it myself.
I purchased a piece of software that would work for any car that would read only the generic codes (as our other car could benefit from it too) via the OBD2 connector. There were several fault codes in the memory (I was amazed that the workshop did not find these) of which I started to research. The codes of importance were:
P0302 Cylinder misfire detected - Cylinder number 2
P0304 Cylinder misfire detected - Cylinder number 4
P0300 Cylinder misfire detected - Random cylinders
P0040 BMW - O2 Sensor Signal Wires Swapped

Now, when I took these codes back to the garage, they replaced all the O2 sensors and my cat convertors as well. Not sure if they were damaged or not, but its nice to know they are new on a 100,000km vehicle (cost was covered by warranty)
Got the car back after that and the symptoms gradually returned again - exactly the same codes.

So car was sent to the local BMW dealership who pulled the engine apart and claimed that the entire top end had to be replaced - apparently $14,000 worth. I knew warranty wasn't going to cover that much, so the car was put back together (charged to the warranty again) and handed over to a European car specialist.
They replaced a lot of things including the O2 sensors (again), camshaft sensor - the other items don't come to mind unfortunately.
They gave the car a good test and thought they had it sorted - but the problem came back again. It became obvious that every time we cleared the fault codes from the ECU (using DIS, not my generic OBD2 program), the problem would disappear for a few days, then come back.

One of the comments from the mechanic was that he tested compression and it was perfect with the engine, so he highly doubts it is a mechanical issue - he believed it was an electrical issue as it comes and goes.
So the Euro-specialists sent my ECU off to Sydney where it could be tested independently of the engine, and it came back OK.
The car was also sent off to a fuel injection specialist who could not find the fault either.

We were ready to throw in the towel, reset the codes using DIS and try to trade it in as quick as possible, but the codes reappeared just as the car was being scrutinized for trade-in value.

Leaving us no choice, I decided to gather whatever resources I could find (the Euro-specialist let me take home his DIS setup a few times - good bloke!) to try and nut out the problem.

My first port of call was the O2 sensor issue - surely this would be an easy fix? I located the offending connectors and swapped them around, reset fault codes and 2-3 days later a different O2 code came up saying the other two connectors were swapped around. So I swapped those, reset the fault codes and 2-3 days later the original O2 error came up again.
So that CLEARLY wasn't the cause for the O2 error. A lot of my research found that O2 errors can be caused by other things going on in the engine before the O2 sensors (which is pretty much anything) so my attention was now on the misfire fault codes.

I spent a good 4 weeks (part-time, as I work full-time) trying all sorts of ECU resets, swapping ignition coils, sparks plugs, injectors etc. to see if the misfires changed cylinder numbers - which they didn't.
In the end, I used a combination of resources:
Part Directory: realoem.com/bmw/
This helped identify exact locations of parts and their part numbers for research

BMW Workshop Manual: bmw.workshop-manuals.com
For obvious reasons

BMW Wiring Diagram Manual: bmw-planet.com/diagrams/release/en/e87/index.htm
Again, for obvious reasons

I went through the electrical wiring and examined each wire relating to the misfiring cylinders and comparing them to the cylinders that were not misfiring.

I eventually came across one half of the solution - the injector connectors for cylinders 3 and 4 were swapped around. I couldn't believe it - 3x car yards and one injector specialist missed this!!! Thank goodness for the wiring diagram manual, oh, and this link to decipher what the colour codes are: http://www.bimmerfest.com/forums/sho...d.php?t=494164

So I swapped these around, reset the fault codes and started the engine - WOW! She was really purring now, I had never felt such power at low revs from this car before. Smoothness returned and the engine shake was completely gone.
Feeling very happy with myself I announced to the wife (it was her car after all) that I had resolved the issue.

2-3 days later the engine light came back on, but there weren't any of the usual symptoms associated with it.
I cursed for about 5 minutes, connected my diag software and noticed the misfire codes and O2 codes were still appearing.....but wait.....one of the misfire codes moved from cylinder 4 to cylinder 3.....that's when I had an AHA! moment.

Using the OEM Parts website, I could see that cylinders 2 & 3 shared the same downpipe, and thus, the same O2 sensors - meaning that maybe they really are swapped around (or perhaps broken...again). I got under the car and spent a good hour just looking at the connectors to determine if there was anything obvious to help determine which connector went where, and whether they are currently correct.
I finally saw a tiny plastic clip on one of them, but not on the other (these are the connectors that are somewhat rectangular, not the round ones - as there are 4x O2 sensors with 2x different connectors) so I tried to find where the clip would have connected to the engine/chassis - and there I found it...proof that the connectors were indeed swapped around. I swapped them back, reset fault codes and it has now been a month of driving without a single hiccup. The car drives beautifully and has plenty of power to boot now.

I suppose the biggest lesson I have learnt is that the fault codes are there to assist your diagnosis, not to replace you actually doing the diagnostics yourself. In my case, I had multiple causes to my fault codes and they needed to be rectified before any of the codes would go away - so don't expect to fix fault codes one-by-one if you have multiple, they will likely be inter-related.

I may have missed some details in this post so feel free to reply to this and I'll try to answer any questions you may have.

The Rug_a_
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Old 06-06-2014, 05:00 PM
sudnut sudnut is offline
Registered User
Location: World's Biggest Island
Join Date: Dec 2010
Posts: 12
Mein Auto: E46 318i
Brilliant write up, thanks. I am dealing with similar symptoms on the N42 engine in my 318i e46. Just goes to show that you can't dismiss the possibility that the car is in perfect condition but something has been reassembled incorrectly. Multiple faults also make it exponentially more difficult.
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e87, misfire, n45, o2, swapped

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