BMW 29D0 identifies cylinder 4 misfire on the installed DME; it does not automatically mean coil 4 failed. On an E9x N51/N52 328i, preserve smooth-running values and cold/load conditions, label plug and coil 4, and make one controlled ignition comparison. If the fault stays on cylinder 4, test injector contribution, local intake leakage, compression/leak-down and cam timing. Do not apply N54 injector bulletins merely because they mention the same numeric misfire code.
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 cold and hot and keep removed plugs labeled. Deposits, wet fuel and excess gap support different paths.
Move one ignition component only and watch whether the fault follows.
Do not borrow an N54 repair
BMW's N54 bulletin lists 29D0 among injector-related cold-start faults, but a stock 328i N51/N52 is a different engine and injector system.
Use the VIN-specific ISTA injector and fuel tests for the installed engine.
Prove mechanical contribution
If ignition and fuel tests do not move the fault, perform relative compression, leak-down and synchronization checks.
Verify cylinder 4 smooth-running values under the original condition with no 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
The 328i badge covers very different powertrains. North American E9x cars generally use naturally aspirated N51/N52 six-cylinder engines, while F30/F31 cars use turbocharged N20/N26 four-cylinder engines; other markets and swaps vary. Read VIN, chassis, engine designation, DME identification and the complete BMW fault text before using any test. A four-digit BMW fault is not safely interchangeable with a generic P-code or a fault from a 335i.
- 29D0 is cylinder 4; 29D1 is cylinder 5 in the relevant DME family.
- N51 SULEV and N52 fuel/EVAP hardware can differ.
- The N54 cold-start injector bulletin is not a stock 328i bulletin.
- A flashing MIL or severe shaking requires stopping.
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 Technical Information SystemOfficial VIN-, engine- and ISTA test-plan information↗02BMW owner's manualsOfficial VIN/model operating, warning and maintenance information↗