Start with the code and the symptom
A misfire code or a flashing check engine light points at ignition or fuel, while a smooth-running engine that shakes the whole car at idle in Drive points elsewhere. The repair pages describe each cause with its own tell: misfire codes with a service reminder for plugs, a misfire that follows one injector, VANOS codes with hesitation, or misfire codes with good plugs and coils for carbon. This guide is built from reported repairs, not a diagnostic flowchart, so confirm the cause with a scan before replacing parts. Plug-specific detail is in the BMW Spark Plugs and Ignition Coils: When to Replace and What It Costs.
Spark plugs and ignition coils
Misfire codes, rough idle and a spark plug service reminder are the signs on the N55 page, and several commenters say coils are not a wear item, so replace them only if you get misfires. A shop owner says they bill 1 to 2 hours depending on four or six cylinders, and for a DIY job the strut brace and intake pipe may need to come out. See Spark Plugs & Ignition Coils (N55).
On the B58 and B48 the plugs are the scheduled item and coils only need replacing if they fail or test bad. Dealer plug quotes are common in the sources, so an independent quote is worth getting for comparison. See Spark Plugs & Ignition Coils (B58 / B48). Oil in the spark plug wells from a leaking valve cover can also cause misfires (Valve Cover / Valve Cover Gasket, B58 Valve Cover Replacement).
Fuel injectors
On the N55, owners describe a misfire on one cylinder that follows the injector, a rough idle with a fuel smell, and smoke or a hard start after sitting. EU5 injectors need injector codes entered in the DME while EU6 kits generally do not, and owners put the labor at about 3 hours for all six. Ask whether the quote is for one injector or all six. See N55 Fuel Injectors.
On the N54, misfires on several cylinders are an injector sign alongside a long crank and fuel pressure codes, and commenters recommend logging fuel pressure before replacing the high pressure pump. See N54 High Pressure Fuel Pump & Injectors.
VANOS solenoids and intake carbon
Rough idle, stalling at stops, hesitation at highway speed and VANOS codes such as 2A82 and 2A87 point to the VANOS solenoids on the N52 and N54. Swapping the two solenoids is a quick way to see whether the fault moves from intake to exhaust before buying parts, and the job takes about 15 to 20 minutes per an owner. See VANOS Solenoids.
Direct-injection engines build up carbon. The signs are rough idle, cold-start misfires, hesitation and misfire codes with good plugs and coils. N55 owners report much less carbon than N54 at the same mileage, so many skip it on the N55, and commenters say a fair independent price is well below the quotes that suggest a shop does not want the job. See Carbon Cleaning / Walnut Blasting.
Sensors and engine mounts
O2 sensor codes can bring an erratic idle and poor fuel economy. Confirm which sensor (bank and position) the code points to, and use a Bosch sensor rather than a generic one per forum advice. See O2 / Oxygen Sensor Replacement.
If the vibration is at idle in Drive, with a clunk when shifting into gear or an engine that moves a lot when revved, the mounts are the suspect, and the 335i mounts are fluid-filled and wear sooner. The page also warns to get a second opinion, because in one thread an independent shop confirmed the mounts a dealer flagged were still fine years later. See Engine Mounts.