What P0306 actually means in plain English
P0306 fires when your BMW's engine control unit detects a misfire in cylinder 6. A misfire happens when the fuel-air mixture in that cylinder either fails to ignite or burns so poorly that combustion is incomplete. Think of it like a spark plug that won't spark, a coil that's dead, fuel not reaching the cylinder, or compression so low the mixture won't light at all.
Your ECU monitors each cylinder constantly by watching the crankshaft sensor. When it sees cylinder 6 fire slower or weaker than expected - or not at all over a certain number of cycles - it logs P0306 and typically lights the check engine lamp. The engine might run rough, shake on acceleration, or feel like it's limping. Some owners don't notice anything obvious until the code appears during a scanner pull.
This code sits in the ignition category because the most common culprit is spark delivery - either a worn plug or failed coil pack. But don't assume that yet. I've chased P0306 on customer cars and found fuel injectors, compression loss, and even air intake leaks causing the fault.
How to diagnose P0306 step by step
Step 1 - Pull full scanner data before touching anything. Connect a proper BMW scanner - not a basic OBD2 reader - and log live data. Watch cylinder 6 specifically during idle and light acceleration. Check fuel trim numbers, oxygen sensor readings, and whether other cylinders show misfires too. If cylinders 1 through 5 and 6 are all misfiring, you've got a different problem - likely ignition timing, fuel pressure, or a vacuum leak. If it's only cylinder 6, you're hunting a cylinder-specific cause.
Step 2 - Visual inspection of ignition components. Pop the engine cover and look at the coil pack sitting directly on top of cylinder 6's spark plug. Look for cracking, oil seeping into the connector, or corrosion. These coils fail constantly on higher-mileage BMWs. While you're there, check the spark plug wire or connector is seated tight. Then unplug the coil and carefully pull the plug out with a socket and extension - note the color and gap. A black, fouled, or gapped plug is your answer. Cost to fix: thirty bucks and twenty minutes.
Step 3 - Compression test on cylinder 6 only. If the coil and plug look fine, a compression test takes ten minutes and rules out a stuck valve or worn piston ring. Unscrew all six plugs, remove the fuel pump fuse to prevent injection, crank the engine with the compression gauge in cylinder 6. Compare to the factory spec and to your other cylinders. Low compression means internal engine damage - that's a shop job. Normal compression keeps you moving forward in the diagnosis.
Step 4 - Check fuel injector pulse and resistance. A bad fuel injector in cylinder 6 will cause exactly this code. With the scanner connected, command cylinder 6's injector on and off while monitoring fuel trim correction. Does it respond? Use a multimeter to check the injector's coil resistance - should be around 11-14 ohms depending on your BMW's generation. An open or shorted injector will show wrong resistance and needs replacement.
Step 5 - Smoke test the intake. A vacuum leak between the intake manifold and the fuel rail or cylinder head on the number 6 side will allow unmetered air past the sensor, leaning out the mixture and causing misfire. A smoke machine costs nothing to run at a shop but can be hard to do at home. If compression and coil and plug all check out, a smoke test or even just careful visual inspection around the number 6 intake gaskets is worth the time.
DIY fix for P0306
This code has a DIY difficulty rating of 1 out of 5, which means you can fix it at home with basic tools in most cases - but only after diagnosis.
Replacing the spark plug: Remove the engine cover (usually held by one or two clips), unplug the coil pack from cylinder 6, and pull the coil straight up. Use a 16 mm spark plug socket on a ratchet wrench and carefully unscrew the old plug. Check the gap on the new plug (should already be gapped to spec from the parts store), thread it in by hand first to avoid cross-threading, then torque snug with the socket - don't crush it. Reinstall the coil, clip the cover back on. Clear the code with your scanner and test drive.
Replacing the coil pack: Even easier. Unplug the connector - don't force it; press the tab on the side. Then grab the coil with both hands and pull straight up. It comes out in one motion. New coil goes in the same way - press until you feel the clicks, plug the connector back in. Restart and clear codes.
Fuel injector replacement: This is technically a DIY task on most BMWs but requires more care. You'll need a fuel pressure relief valve, a 10 mm socket for the mounting bolt, and steady hands. If you've done basic engine work before, it's doable. If you haven't, a dealership or independent shop takes thirty minutes and charges labor. The part itself costs $80 to $200 depending on generation.
Compression testing, smoke testing, and electrical diagnostics belong in a shop unless you own a scanner and compression gauge already.
When P0306 comes back after repair
If the code returns after you've replaced the plug, coil, or injector, suspect one of three things:
One - you fixed the wrong component. A plug and coil were both bad, but you only swapped the plug. Replace the coil next and clear codes again.
Two - the underlying cause is still there. A vacuum leak or low compression will kill a new injector or coil just like it killed the old one. Scan data and compression test again. Don't throw parts at it twice.
Three - intermittent fault, likely wiring or connector. If the code is sporadic and you've verified spark, fuel, and compression all check, the issue might be a corroded connector pin or frayed wire to the coil. Inspect the connector contacts closely. If the wire is damaged, replace the entire coil pack harness.
My take on P0306
In five years wrenching BMWs and one year at a dealership, I've seen P0306 pop up regularly on higher-mileage cars - usually it's the coil pack on its way out. The good news is cylinder 6 misfires are almost never expensive to fix if you catch them early. The bad news is ignoring it wrecks your catalytic converter, which is a five-figure repair on newer models.
Don't assume it's the plug. Scan first, look at the coil, then decide. Ninety percent of the time you're replacing a coil, and that's a five-minute job and a thirty-dollar part.
Severity-wise, I rate this as yellow - drive carefully to a shop or diagnostics point, but you're not in emergency territory. The engine won't seize. That said, don't ignore it for weeks. Get it scanned, do the visual check, and fix the root cause before your oxygen sensors start reading bad and your cat clogs up.
Need more detail on how to read your scanner? Check out our best BMW scanner guide and our coding walkthrough. And if you want to understand fault codes as a whole, read our fault code explainer. Want to search more codes? Head back to the fault code tool.