Misfires from failed ignition coils are a common fault on BMW’s N and B inline gasoline engines. The coil generates the high voltage for the spark at the plug; when one coil gives up, the fault memory logs misfires for a specific cylinder, and a flashing malfunction indicator lamp (MIL) warns that fuel is actively burning in the catalytic converter. Below are the symptoms, the causes, the diagnostic procedure, and the prevention rules.
Which Engines and Models Are Affected
The “bad coil” diagnosis applies to every BMW inline gasoline engine. N54, N55, and B48 owners run into it most often. The procedure applies in particular to the 3 Series E90, E91, F30 and the 1 Series E81.
Confirm applicability to a specific car using the 17-digit VIN.
Symptoms
- The engine runs on 3 or 5 cylinders.
- The MIL flashes during an active misfire.
- Vibration and shaking at idle.
- Jerking under load, loss of power.
- The car drops into limp mode.
- Fault memory shows 29F1–29F6, 29CB–29D2.
Causes
One of three factors is usually to blame. Secondary winding aging: over time the winding and its insulation degrade, the breakdown voltage drops, and the coil stops jumping the spark gap reliably.
Moisture in the coil well: condensation or seepage accelerates contact corrosion and sets up a breakdown. Worn spark plugs: a widened gap demands higher voltage, the coil runs at its limit, and a weak winding quits sooner.
Diagnostics
The sequence: pull the fault memory and the misfire counter for each cylinder; swap coils between cylinders — if the code “moves” with the coil, the coil is at fault, not the cylinder; inspect the plugs for electrode wear and oil film.
Step-by-Step Check
- Pull the code and identify the cylinder.
- Swap the problem cylinder’s coil to another cylinder.
- Inspect the spark plugs and replace if needed.
- Clear the fault memory and take a test drive.
Repair
Install an OEM coil or one from Bosch/Delphi. At high mileage, the spark plugs go in along with the coils — NGK or Bosch to BMW specification. A coil is a wear item, a catalytic converter is not: you can’t ignore a flashing MIL — unburned fuel destroys the converter’s honeycomb.
Prevention
- Spark plugs every 37,000 miles on gasoline engines.
- Inspect the coils whenever you replace the plugs.
- Past 62,000 miles, replacing the entire set is justified — a wave of failures is likely.
Fault Code Table
| Code | Meaning |
|---|---|
| 29F1–29F6 | Misfires on cylinders 1–6; 29F1 — coil for cyl. 1, 29F2 — cyl. 2 |
| 29CB–29D2 | Misfire code group |
| 29CD | Misfire detected |
| 2C77 / P0135 | Oxygen sensor (heater circuit) |
How the Engine, Symptom, Code, and Part Connect
- Engine: BMW B48.
- Symptom: rough idle, the engine shakes in place.
- Problem: electric water pump failure.
- Code: 29CD — misfire detected.
- Service: Valvetronic diagnostics.
FAQ
Replace one coil or the whole set at once?
Past 62,000 miles, replacing the complete set is justified — a wave of failures is likely.
Related Articles
- 29CD — misfire detected
- 29F1 — cylinder 1 coil
- 29F2 — cylinder 2 coil
- BMW B48, N54, N55 — engine profiles
- BMW: rough idle
- BMW: check engine light on
- BMW: jerking — causes, diagnostics, repair
- BMW: oil leaks, vibrations, no-start
- Valvetronic diagnostics
- Misfire diagnostics
- Oxygen sensor diagnostics (2C77 / P0135)
- Electric water pump check
- Thermostat check
- BMW Zündspulen — replacement and spark plugs
- BMW: valve cover gasket — fixing the oil leak
- BMW: water pump — electric and mechanical
- BMW Lambdasonde — control and diagnostic sensors
- BMW Valvetronic — variable valve lift
- BMW KGE / PCV — membrane, oil separator, limp mode
- BMW VANOS — operation, faults, repair
- BMW 3 Series E90, E91, F30; BMW 1 Series E81
What Shops See in Practice
A single coil failing between 37,000 and 75,000 miles is routine business on the N and B engines. Most cars roll in with a flashing light and a shake; swapping coils confirms the diagnosis in one visit. At high mileage, plug replacement is usually bundled with the coils.
Bottom Line
A flashing MIL with misfires is the signal to stop driving immediately and deal with the ignition. The quick check relies on swapping coils; past 62,000 miles, it makes more sense to replace the full set along with the plugs.