[Hxm99]

Hello all
as you may have already guessed from the title I'm having some trouble diagnosing a little something I presume to be an issue(not sure if this is just normal behavior for this engine!). The car is a E93 325i(N53 engine) after a cold start the rev counter fluctuates very very slightly sometimes, upon scanning the car with carly, it throws two codes relating to the Nox sensor and the Nox cat ageing code. The car seems to drive fine and averages about 40 mpg when cruising at 60mph on the motorway( I assume that's good economy for this car?). I dont wanna end up with a major issue down the line by not attending to any potential existing issue, therefore I request you all to watch 20 second clip I've uploaded(link provided) and help a brother out. Can a faulty Nox sensor cause slight fluctuations? or is this normal behavior for this engine or this an injectors on its way?
Note- spark plugs were last replaced in 2017

[Markos ZimZimma]

Others who are more mechanically minded may chime in with other opinions – and I get how we all want a rock solid idle – but I'd not be sweating about that at all. You've noted it and recorded it. Keep an eye on it each month and see if it changes. In my experience with very, very small issues I've learnt to wait until they become more obvious – if they do at all – and it will be easier for mechanics to work out what's wrong when the issue is more pronounced or obvious. As you've scanned it and everything is clear I think just keep an eye on it once a month to see if it changes for the worse.

[Hxm99]

Originally Posted by Markos ZimZimma

Others who are more mechanically minded may chime in with other opinions – and I get how we all want a rock solid idle – but I'd not be sweating about that at all. You've noted it and recorded it. Keep an eye on it each month and see if it changes. In my experience with very, very small issues I've learnt to wait until they become more obvious – if they do at all – and it will be easier for mechanics to work out what's wrong when the issue is more pronounced or obvious. As you've scanned it and everything is clear I think just keep an eye on it once a month to see if it changes for the worse.

Thank you!

[basim07]

Long time N53 owner here

RE the NOX sensor, it will be ageing faulty, and in the long term that will affect economy, but also can have an affect on rough idle amongst other things, you can do more detailed analysis on it using INPA and can tell how well its functioning etc. Also you can reset / readapt the sensor with ISTA also.

Whether that's the cause of the rough idle - yes it possible. But also valve cover cracks, leaky injectors, worn plugs, worn coilpacks, caked intake valves can also cause it!

Seeing as you have no other codes or problems, I wouldn't worry about it too much. If you do wanna delve in it, get INPA and monitor the fuel trims for each bank to see if there's any issues, will tell you if you have a small vacuum leak or leaky injector. Alternatively pull the plugs out and have a look at the health of them, a sooty plug is always a good indication a cylinder isn't running at its best.

At 4 years old I'd probs change your plugs, I changed by 3 years ago and few weeks ago changed them all again.

[Hxm99]

Originally Posted by basim07

Long time N53 owner here

RE the NOX sensor, it will be ageing faulty, and in the long term that will affect economy, but also can have an affect on rough idle amongst other things, you can do more detailed analysis on it using INPA and can tell how well its functioning etc. Also you can reset / readapt the sensor with ISTA also.

Whether that's the cause of the rough idle - yes it possible. But also valve cover cracks, leaky injectors, worn plugs, worn coilpacks, caked intake valves can also cause it!

Seeing as you have no other codes or problems, I wouldn't worry about it too much. If you do wanna delve in it, get INPA and monitor the fuel trims for each bank to see if there's any issues, will tell you if you have a small vacuum leak or leaky injector. Alternatively pull the plugs out and have a look at the health of them, a sooty plug is always a good indication a cylinder isn't running at its best.

At 4 years old I'd probs change your plugs, I changed by 3 years ago and few weeks ago changed them all again.

Thank you sir!