BMW fault code P0301 / 29CD (N54, N55): cylinder 1 misfire — the factory test path and the honest causes
P0301 (BMW hex 29CD) is a misfire counter tripping on cylinder 1 only. Here's the factory move-parts test, the cat warning the doc carries, and what actually fails.
P0301 is the same combustion-consistency measurement as P0300, but the counter that tripped belongs to one cylinder — cylinder 1 (BMW hex 29CD). That single fact collapses your search space: one hole is either not sparking, not injecting, or not compressing.
What the factory document says
Factory service document29CD / P0301 — Combustion misfires, cylinder 1 (factory fault doc)Se abre en el conocimiento de servicio de tu cuenta — requiere iniciar sesión.The monitor runs between roughly 400 and 6,400 rpm, counting misses within 200 crankshaft rotations weighted by engine speed and load. The factory test sequence is the model of efficiency: visual inspection of spark plugs and ignition coils; if OK, switch positions — if the defect moves with the parts, replace them; then check ignition cables and wiring harness; then check the injector valve and the fuel system including the high-pressure pump. One more factory step most people skip: inspect the catalytic converter for damage — unusual colors on the casing, monolithic structure checked with an endoscope — because sustained misfire dumps raw fuel into the cat and cooks it.
What usually actually causes it
Community experience on N54/N55, paraphrased and ranked:
- Coil or plug on that hole. The move-parts test exists because it's usually the answer. A single-cylinder misfire that appeared after a plug change is a cracked porcelain or a coil not fully seated.
- Injector — index-mismatched or worn piezo injectors on N54 misfire on one cylinder, often when cold. The DME's injector calibration can't fix a mechanically tired injector.
- Valve cover gasket leak letting oil pool around one coil boot — arc to ground, single-hole misfire, recurring after coil replacement until the cover is done.
- Low compression on that hole — burned exhaust valve or ringland. The move-parts test fails to move the miss; that's your cue to do a compression or leak-down test before buying more ignition parts.
If multiple cylinders are counting, you're on the P0300 page — different cause bias.
The fix path
- Run the factory move-parts test: plugs and coils first, cheapest and most likely.
- Misfire that follows no part and persists cold → log it: the per-cylinder misfire counters plus fuel trims point ignition vs fueling vs compression fast.
- Coil well oily → valve cover and gasket, not another coil.
- Fixed the misfire but the CEL returns as P0420? The cat inspection step above is why — check it before assuming sensor drift.
How do I run the factory move-parts test?
Visual inspection of plugs and coils first; if they look OK, swap the suspect cylinder's coil and plug with a healthy cylinder's. If the defect moves with the parts, replace them — then the harness, then the injector and fuel system including the high-pressure pump.
Why does the factory test include the catalytic converter?
Because sustained misfire dumps raw fuel into the cat and cooks it — the document's own step is to inspect the casing for unusual colors and check the monolithic structure with an endoscope. It is the difference between an ignition fix and a four-figure catalyst bill.
The misfire stayed after plug and coil swaps — what next?
Work the factory order: wiring harness, then the injector on that hole, then compression or leak-down before any more parts. A miss that follows no part is pointing below the head gasket line.
- ReferenceBMW fault code P0300 / 29CC (N54, N55): random misfires — what it means, what actually causes it
- ReferenceBMW fault code P0420 / 29F6 (N54, N55): catalytic converter efficiency below threshold — what the code actually measures
- TroubleshootingMisfire triage the factory way: coils, plugs, injectors, compression — in order
- GuidesSpark plugs and coils: the N54's maintenance heartbeat, factory torques and gotchas
- GuidesWalnut blasting and intake valve cleaning: the GDI carbon job, factory view
- GuidesReading your datalog: RPM, boost, AFR
- ReferenceThe code reader's companion: tools, codes, and what the data means