2011 BMW 328i Rough Running and P1727 Code with Battery Drain
I bought a 2011 BMW 328i that already had a crankshaft position sensor code (P1727) and it’s been running very rough. I replaced the crankshaft position sensor, ignition coils, and spark plugs, but it still runs rough and the battery kept draining quickly. After that I installed a new alternator and a new battery, and I’m still dealing with the same rough running, the P1727 code is still showing, and the battery continues to drain to the point that I have to jump it almost every time I try to start it. What could be causing this?
The persistent P1727 crankshaft position sensor code and rough running after replacing the sensor, coils, plugs, battery, and alternator point strongly to a wiring or mechanical issue related to the crank sensor circuit. The battery drain despite a new alternator and battery suggests a parasitic draw, possibly from damaged wiring or a short.
Most likely culprits
- Crankshaft position sensor wiring harness: Damage, corrosion, or poor connection can cause the sensor to send intermittent or no signal, triggering the code and rough running.
- Reluctor (tone) ring on crankshaft: If damaged or dirty, it can cause the sensor to misread position, causing rough running and the code.
- Parasitic battery drain from wiring fault: A short or stuck relay related to the engine management or sensor circuits can drain the battery even with new parts.
How to narrow it down
- Scan for all stored codes beyond P1727 to see if other related faults exist; multiple codes can point to wiring or module issues.
- Visually inspect the crankshaft position sensor wiring and connector for damage, corrosion, or loose pins; wiggle test while monitoring sensor signal if possible.
- Check the reluctor ring on the crankshaft for missing teeth, cracks, or heavy rust; this requires removing the sensor and inspecting the ring.
- Perform a parasitic draw test with a multimeter set to amps on the battery negative terminal with the car off; anything over ~50mA is excessive and needs tracing.
- Check sensor signal with an oscilloscope or scan tool live data to confirm if the crank sensor is producing a clean, consistent signal.
- Verify charging system voltage and current output with engine running to rule out wiring issues causing battery drain despite new alternator.
Possible parts
- Crankshaft position sensor wiring harness or connector repair kit
- Reluctor ring (if damaged)
- Battery cables and grounds (if corroded or loose)
- Fuses and relays related to engine management circuits
A few questions
- Is the P1727 code the only code showing or are there others?
- Have you checked the wiring and connectors for the crankshaft position sensor for damage or corrosion?
- Is there any noticeable parasitic battery drain when the car is off?
- Has the reluctor ring on the crankshaft been inspected for damage or missing teeth?
- Does the rough running occur at all RPMs or only at idle/startup?