P0300 – Random/Multiple Cylinder Misfire Detected (BMW N54)
Fault code P0300 means "Random/Multiple Cylinder Misfire Detected". It is common on N54 (135i, 335i, 535i, X6 35i), N55 (M135i, 335i, 435i, 535i, X3/X4 35i) engines. Below are the real-world causes, step-by-step tests and OEM part numbers.
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P0300 quick summary
| Fault code | P0300 |
|---|---|
| Meaning | Random/Multiple Cylinder Misfire Detected |
| System | Powertrain (Engine & Transmission) |
| Vehicles | BMW |
| Common on | N54 (135i, 335i, 535i, X6 35i), N55 (M135i, 335i, 435i, 535i, X3/X4 35i) |
| Severity | High |
| Can I drive? | Only gently. If the engine shakes heavily or the Check Engine light flashes, stop – misfires overheat the catalytic converters. |
| Repair cost | €30 – €1,500 |
Symptoms of P0300
- Engine shakes or stutters under load, often in higher gears
- Check Engine light, 'drivetrain malfunction' message
- Reduced power
- Rough idle on a cold start
- BMW codes 29CD–29D2 (misfire cylinder 1–6) stored as well
Most common causes of P0300 (most likely first)
Mechanic's Pro Tip
On the N54/N55 the order of suspects is almost always: spark plugs, ignition coils, then injectors. Swap the coil of the misfiring cylinder with a neighbour – if the misfire moves, it is the coil. Tuned engines need much shorter spark plug intervals. On the N54, a misfire that follows the injector after a swap points to a failing piezo injector.
1.Worn spark plugs (very common, especially on tuned engines)
Most likely2.Faulty ignition coil(s)
3.Failing piezo injector (N54) or leaking injector
4.Low fuel pressure (high-pressure pump, low-pressure pump) – BMW code 29DC / 29D9
5.Vacuum / boost leak or carbon build-up on the intake valves
Step-by-step diagnostic guide
- 1
Read the DME fault memory with ISTA or a BMW-capable scanner; the BMW misfire codes 29CD–29D2 show which cylinders are affected.
- 2
Check the spark plugs: gap, wear and colour. Replace them if they are due.
- 3
Swap the ignition coil of the affected cylinder with another cylinder and see whether the misfire moves.
- 4
If the misfire stays with the cylinder, swap the injector or check the injector coding / adaptation values.
- 5
Check low- and high-pressure fuel values in live data, especially if fuel-related BMW codes are stored.
Frequently asked questions
Should I replace all ignition coils at once?
If one coil has failed and the others are old, replacing the full set avoids repeat visits. If the others are fairly new, replacing only the faulty one is fine.
How much does it cost to fix misfires on an N54/N55?
A set of spark plugs costs a few dozen euros in parts, an ignition coil roughly 25–200 €. Piezo injectors are much more expensive – often several hundred euros each – and must be coded to the DME after replacement.
Related fault codes
This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.