Cylinder 4 Misfire Detected

Cylinder 4 misfire is typically an ignition coil failure (Denso). Swap the coil from cylinder 4 to cylinder 2 -- if the misfire follows, replace the coil. Cylinder 4 runs hottest on the 2AR-FE and coils fail there first. Also replace the spark plug (NGK) and check the valve cover gasket for oil leaking into the plug tube. Confirm the exact part for your year and engine with a retailer's fitment lookup or your VIN before ordering - these applications split by engine and build date.

Cylinder 4 is an AFM cylinder on the GM 5.3L V8. A collapsed AFM lifter is the most common cause. Check lifter preload with the valve cover removed. If collapsed, replace all 16 lifters and consider an AFM delete kit (DOD delete) to prevent recurrence. Your dealer can confirm whether an open service bulletin or warranty extension applies to your VIN. An AFM/DFM delete also needs an ECM tune, and because cylinder deactivation is part of the truck's emissions-certified configuration, deleting it can affect powertrain warranty coverage and road-legal emissions compliance - check your local rules before going that route.

Perform a coil swap test first. If the coil is good, check valve lash on cylinder 4 -- Honda K24 engines require periodic valve adjustment. Tight exhaust valves cause misfires when hot. Spec is 0.010-0.012 inch exhaust. Also check fuel injector resistance (should be 11-13 ohms) and clean with Seafoam if sticking.

Cylinder 4 on the boxer engine is the driver-side rear cylinder. Heat cycling causes ignition coil cracks (Denso). Also check for oil fouling from valve stem seal leaks -- common on the FB25. If compression is low on cylinder 4, suspect a leaking head gasket which is a known Subaru issue on earlier models. Confirm the exact part for your year and engine with a retailer's fitment lookup or your VIN before ordering - these applications split by engine and build date.