[daxfit]

HI there guys, apologies if I have posted this in the wrong place.

Hoping to get a bit of advice on an issue I've been having with my 2009 120i.

So basically, I haven't owned the car long, initially all was fine however about 2 weeks after purchase after the car had been sat for a few days in the cold, I had the flashing engine light and the engine was juddering a bit. I arranged for the car to be looked at the garage right away.

Once warmed up the judder subsided and the car seemed fine (engine light also turned off), I also don't think I have any loss of power or misfire.

The garage plugged in the car and looked at the error codes to see that there was an issue with the NOX sensor and desulphurization.
After a change of the Nox sensor and a hefty bill the car was returned and driving fine and smooth.

Left the car for a day or so (during very cold weather) and hopped back in just to get the exact same problem as before. Juddery engine flashing engine light.

I've been doing some research and it seems as if the Nox sensor change may have been unnecessary and that its more likely to be the fuel injectors and/or Coil pack/spark plugs.

I plan to replace these however due to limited funds was hoping to do so in stages.
Just wondering what the best order to do so would be?
The lower price of replacing the coil packs and spark plugs first is most attractive and then fuel injectors when I have a bit more in the bank. However, if I replace the coil packs but have a leaky injector, can this just cause further damage to the new ones installed?
I have read that you need to replace all 4 Injectors at once, however this isn't financially viable at the moment (unless I use serviced, second hand injectors that is, any thoughts on this?).

Just wanted to find out if any one had any advice on how to approach the situation!
Cheers!