🚨 Diagnostic Profile
Vehicle Make: BMW
Technical Description: Misfire Cylinder 4 With Fuel Cut-off
This fault indicates that the Engine Control Unit (DME) has detected irregular combustion events specifically on cylinder 4. Once a certain misfire threshold is exceeded, the vehicle triggers a fuel cut-off to that cylinder to prevent catalytic converter damage from unburnt fuel entering the exhaust stream.
🚨 Repair Difficulty Tier
Difficulty Rating: Intermediate
This repair requires a diagnostic scan tool capable of reading live engine data (misfire counters, fuel trims) and potentially a multimeter to test ignition coil primary/secondary resistance. While component replacement is mechanically straightforward, accurate diagnosis requires interpreting live sensor data to distinguish between an ignition, fuel delivery, or mechanical (compression) issue.
🛠️ Resolution Steps
Step 1: Preliminary Inspection
Begin by visually inspecting the ignition coil and spark plug for cylinder 4. Check the wiring harness connector at the coil for corrosion, bent pins, or looseness. Inspect the spark plug for carbon fouling, oil contamination, or excessive electrode gap wear. Also check the intake manifold and vacuum lines near cylinder 4 for cracks that could cause a lean misfire condition.
Step 2: Diagnostic Verification
Connect a scan tool and pull live data streams, paying close attention to the misfire counter for cylinder 4 specifically versus other cylinders. Monitor short-term and long-term fuel trims to determine if the engine is compensating for a lean or rich condition isolated to that cylinder. Swap the ignition coil from cylinder 4 with a known-good coil from another cylinder; if the misfire follows the coil, the coil is faulty. If the misfire stays on cylinder 4, test the fuel injector resistance and perform a cylinder compression or leak-down test to rule out mechanical issues like a worn valve or piston ring.
Step 3: Resolution & Reset Process
Based on diagnostic findings, replace the faulty component: ignition coil, spark plug, or fuel injector as applicable. After replacement, clear the stored Diagnostic Trouble Codes (DTCs) using the scan tool. Perform a test drive while monitoring live misfire data to confirm cylinder 4 now fires consistently with zero misfire counts. Reset any adaptive fuel trim values if necessary to allow the DME to relearn optimal fueling parameters.
📦 Parts, Labor, and Budget Estimates
Required Components/Software: Ignition coil pack, spark plug (NGK or OEM Bosch), fuel injector (if applicable), OBD-II diagnostic scan tool with live data capability.
Average Component Price: Ignition coil: $40–$90 per unit. Spark plug: $15–$30 each. Fuel injector (if required): $120–$250 per unit.
Estimated Labor Hours: 0.5–1.5 hours for diagnosis and coil/spark plug replacement. If fuel injector replacement or compression testing is required, estimate 1.5–3.0 hours total labor.