2014 Malibu with a P0304 and a flashing check engine light.

Flashing CEL = active misfire. Per SAE J2012, a flashing MIL indicates a severe misfire that can damage the catalytic converter. P0304 = cylinder 4 misfire.

Swap strategy (in order):

1. Spark plug first – Remove cylinder 4 plug, inspect for fouling/wear. Swap with cylinder 2 or 3 plug (same type). Clear code, run engine. If P0304 moves to that cylinder, replace the plug.

1. Coil pack next – If plug swap doesn't change anything, swap cylinder 4 coil with another cylinder's coil. Clear code, run. If misfire follows the coil, replace it.

1. Fuel injector last – If above steps fail, swap cylinder 4 injector with another. Listen for misfire transfer.

Why flashing? Unburned fuel is entering the exhaust, overheating the cat. Don't drive it hard until resolved.

Safety note: This is diagnostic work, not repair requiring a license. But if you're not comfortable with engine diagnostics, refer to a certified technician. The flashing light means stop driving ASAP to prevent expensive cat damage.

Clear codes with a scanner after each swap. Test drive 5-10 minutes per step.

The light is flashing because the PCM detects active catalyst-damaging misfire (raw fuel entering the cat). Do not drive it.

Swap Order (Cheapest/Easiest → Hardest):

1. Spark Plugs (Cyl 4 ↔ Cyl 1/2/3): Pull the Cyl 4 plug. Inspect for fouling, cracking, wrong gap, or carbon tracking. Swap with a known good cylinder. Clear codes, run. If code follows plug (e.g., P0301), replace all plugs (ACDelco 41-110 / NGK 6464). Gap: 0.040".
2. Ignition Coil (Cyl 4 ↔ Cyl 1/2/3): If code stays P0304 after plug swap, swap the coil pack. If code follows coil, replace that coil (ACDelco D585 or Delphi GN10372). Torque: 89 in-lbs (10 Nm).
3. Injector (Cyl 4 ↔ Cyl 1/2/3): Requires fuel rail removal, new O-rings, and depressurizing system (remove relay, crank). If code follows injector, replace injector. Lubricate O-rings with clean engine oil.
4. Compression/Leak-down Test: If misfire follows nothing (stays P0304), check mechanicals. 2.5L LKW: Spec ~170+ psi, <15% variance. Check for burnt valve (common on exhaust), blown head gasket (pressurize cooling system), or wiped cam lobe.

Critical 2.5L LKW Specifics:

• Valve Cover/PCV: Check for oil in plug well (leaking valve cover gasket kills coils/plugs). PCV diaphragm fails, causing vacuum leaks/lean misfire.
• Wiring: Inspect coil connector pins for spread terminals/chafing at harness loom near throttle body.
• Mode 6 Data: Check misfire counters ($06) before swapping to confirm only Cyl 4 is missing.

Boss's "Swap it" is correct diagnostic strategy (Isolation), but start with plugs. If compression is low, you just wasted coils/injectors.