Machine · written up April 2026
BMW X1 28i (2017): coils and plugs reset
An ignition coil fires one cylinder's plug. When several are the same age and one is weak, the symptom moves: replace the worst and the next-weakest takes over, and the diagnosis repeats from the beginning.
The question
Replace the whole ignition in one pass or chase the misfire coil by coil: what does a known-good baseline buy the next diagnosis?
Coil-by-coil is the right method when the symptom is sharp: one cylinder, one code, one part. It is the wrong method when the whole set is the same age and the symptom is fuzzy, because then the test is not a test. Swap the suspect coil, the misfire moves to another cylinder, and what has been learned is that the coils are old.
So: one pass, the whole set. Plugs to spec, gapped and torqued; a full set of coils; cabin and engine air filters checked while the cover is already off; codes cleared.
What that buys the next diagnosis
A bisection instead of a search. From a known-good ignition, any misfire that comes back is not ignition, and that single fact removes the largest and cheapest branch of the tree in one move. Fuel delivery, compression, a vacuum leak and the injectors are all still on the table, but they are now on it alone rather than behind a set of coils nobody can vouch for.
It also costs something: this pass throws away the information a coil-by-coil test would have produced. If a single coil was the fault, that is no longer knowable.
What it does not settle
The drive cycles that would close this are not done. Readiness monitors have not been re-run and no post-install scan has been taken, so nothing here says the car is fixed. It says the ignition is known.
- Where it stands
- The whole ignition set went in as one pass. The drive cycles that would close it are not done.
- Next
- Drive a couple of cycles, scan for codes, confirm idle and part-throttle behaviour.
- Tools
- X1 28i (2017) · hand tools