* Camshaft Position Sensor (CMP) or Crankshaft Position Sensor (CKP): These sensors tell the engine's computer the position of the crankshaft and camshaft. A faulty sensor will prevent proper timing and fuel injection, leading to misfires. These are relatively inexpensive and easy to replace, and are a prime suspect when multiple cylinders are affected.
* Wiring: Check the wiring harnesses going to cylinders 1 and 4. Look for broken wires, chafing against metal, or corrosion. Pay close attention to the connectors at both ends. A common point of failure is where the harness goes near the exhaust manifold.
* Spark Plugs (even though you've addressed the coil pack): While you've replaced the coil pack, double-check the spark plugs themselves in cylinders 1 and 4. They could be fouled, damaged, or improperly gapped. It's possible a new coil pack revealed a pre-existing plug issue. Replace them with new, correctly gapped plugs.
* Injector(s): A clogged or faulty injector in cylinders 1 and 4 would prevent fuel from reaching the combustion chamber. You'll need a fuel pressure gauge and possibly an injector tester to diagnose this. A fuel pressure test will tell you if fuel is reaching the injectors and how well. You can swap injectors with other cylinders (a little more involved but doesn't require specialized tools) to see if the misfire follows the injector, isolating the fault.
* PCM (Powertrain Control Module): While less likely, a faulty PCM could be sending incorrect signals to the injectors or ignition system. This is a more expensive and complex diagnostic.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine the wiring harnesses going to cylinders 1 and 4. Look for any signs of damage.
2. Spark Test: Use an inline spark tester to verify spark at the plugs in cylinders 1 and 4. This eliminates any doubt about spark delivery.
3. Check the CKP and CMP sensors: These are easier to replace than the PCM and a common source of this problem.
4. Fuel Pressure Test: This confirms that fuel is getting to the rail at proper pressure.
5. Injector Test (or swap): This should be done *after* checking fuel pressure to isolate a bad injector.
Important Note: Before you start any electrical work, disconnect the negative terminal of your battery to prevent shorts and injury.
If you're not comfortable performing these checks, take your Focus to a qualified mechanic. They have the tools and experience to diagnose the problem efficiently. Describing the symptoms (cylinders 1 and 4 not firing, good compression, new coil pack) will help them narrow down the possibilities.
