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)Opens in your account's service knowledge — sign-in required.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