Emissions

[steveskeggy]

My S55 has just passed the MOT, with a couple of minor advisories (Wheel bearings need tightening, again).
However it only just scraped through on the emissions, which is strange because last year it was way below the fail levels.
I know that the battery is on the way out, but it still starts the car, throws up ESP warnings when I turn the ignition off every now and again, and the odd pre-safety, but nothing when I am driving.
There are no codes current, so I am wondering if the battery could affect the emissions at the time of the test.
I have the feeling that if they had stopped the engine then re-started it would have been a different kettle of fish, but I wasnt there.
They said about a misfire, but it never has with me, its a bit lumpy on tick over, but thats due to the cams.
Got me stumped because I cant re-create it.

 

[grober]

The first thing that comes to mind would be the 2 main sensors connected with [air fuel ratio] mixture control the MAF and lambda sensor. Might be getting a bit long in the tooth or suffering due to low voltage supply- not enough to throw a fault code but enough for your emissions to suffer??

 

[steveskeggy]

No MAF on a 55 engine.
But low voltage to O2 sensors makes sense.