BMW 29D0 identifies cylinder 4 misfire; it does not automatically mean coil or injector 4 failed. Confirm N54 versus N55, preserve smooth-running values and cold/load evidence, label plug/coil 4 and make one controlled comparison. BMW's N54 cold-start bulletin applies only when the listed production/engine group, reproducible cold roughness, wet plug and injector evidence match. Otherwise test injector contribution, pressure, local air and compression normally.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Stop loading to protect catalysts.
What can set 29D0—and how to separate it
The order below is evidence-driven, not a universal probability ranking. Vehicle year, repair history, companion codes and the operating condition in freeze frame change where diagnosis should begin.
| Possible cause | Evidence that supports it | First useful test |
|---|---|---|
| Worn/fouled plug or ignition fault | Cylinder leader and controlled swap follows | Labelled plug/ignition test |
| Injector or fuel-pressure fault | Contribution/pressure mismatch | Injector/fuel tests |
| Intake, purge or boost leak | Trim/smoke pattern | Smoke and purge test |
| Compression, valvetrain or timing fault | Count stays with mechanical weakness | Compression/leak-down/timing |
| Shared voltage or contaminated fuel | All cylinders or post-refueling | Voltage/fuel-quality test |
Preserve cylinder 4 evidence
Capture smooth-running values from cold start through warm idle and the freeze-frame load. Keep plugs/coils labeled.
Move one ignition component only and observe whether the misfire follows.
Apply the N54 injector branch narrowly
On a covered N54 with a wet plug and proven leaking/poor-spray injector, follow the bulletin and current injector compatibility/calibration instructions.
Do not replace a bank or full set on an N55 from an N54 bulletin.
Prove fuel and compression
If the count stays, test rail pressure, injector contribution, local intake sealing, compression and leak-down.
Verify the original cold/load condition without returning 29D0.
Save these scan-data clues
Freeze frame is a snapshot of conditions when the PCM matured the fault. It can distinguish an idle-only problem from a load, cold-start or monitor-completion problem. Clearing first throws that context away.
- Exact VIN, platform, model year and engine sales code
- All confirmed, pending and permanent companion codes
- RPM, load, coolant temperature and vehicle speed
- Relevant commanded and actual scan data
- Recent service, repairs and symptom conditions
- Manufacturer bulletin and current software/VIN status
A practical diagnostic workflow
- 01Identify engine and bank layout
Cylinder patterns differ.
- 02Preserve counters
Save cold/load evidence.
- 03Inspect plugs in order
Deposits direct testing.
- 04Target ignition
Use one controlled test.
- 05Test fuel and air
Graph pressure/trims and smoke paths.
- 06Prove mechanical health
Use compression/leak-down/timing.
- 07Verify original condition
Avoid a flashing-MIL road test.
Do not replace the most commonly named component from 29D0 alone. Confirm the exact engine, preserve evidence and complete the manufacturer test sequence before ordering parts.
Why the model year changes the test
BMW 335i diagnostics divide primarily between twin-turbo N54 E9x cars, single-turbo N55 late-E9x/F30 cars and market-specific variants. The engines share a badge but not boost actuators, injectors, Valvetronic hardware or fault-code families. Confirm VIN, chassis, production date, engine, DME software and modifications before applying an N54 30FF or N55 2C57 procedure.
- 29D0 is cylinder 4 in the relevant N54/N55 DME family.
- N54 injector index/calibration rules differ from N55.
- The N54 bulletin requires reproducible cold-start and physical evidence.
- A flashing MIL risks catalyst damage.
29D0 FAQ
Does P0300 mean all coils failed?+
No.
Can I drive?+
Not with flashing MIL or strong shaking.
Can bad fuel cause it?+
Yes.
Should I replace plugs first?+
Inspect and record them first.
Can vacuum leaks cause it?+
Yes.
Can timing cause it?+
Yes, especially with cam/crank codes.
Why save counters?+
They reveal cylinder and bank structure.
What confirms repair?+
Stable counters under original conditions.
Ford diagnostic references
These documents explain Ford monitor operation. Exact pinpoint tests, connector views and specifications must be selected for the truck's VIN and model year in current service information.
01BMW SIB 13 04 09 — N54 cold-start misfireProduction-bounded N54 branch for reproducible cold-start rough running, wet plugs and proven high-pressure injector faults↗02BMW Technical Information SystemOfficial VIN-, engine- and ISTA test-plan information↗03BMW owner's manualsOfficial VIN/model operating, warning and maintenance information↗