ARCO Mercury 1998-2006 Replacement Ignition Coil

The ARCO IG012 is a replacement ignition coil for 1998-2006 Mercury outboards in the 135, 200, 225, 250, and 300 HP sizes. It replaces Mercury 339-850227 and 339-856991A1. Sierra 18-5187 and CDI Electronics 184-0005 are listed as equivalents. Knowing the horsepower and model year narrows the match, and the Mercury number printed on the old coil confirms it.

The coil turns the low-voltage signal from the ignition module into the high-voltage pulse that fires the spark plug. When a coil weakens, the result is often a misfire on one cylinder, rough running at speed, or hard starting that gets worse as the engine heats up. On engines with a separate coil for each cylinder, a failed coil can be found and replaced on its own. Coils on these engines sit under the cowling where heat and vibration slowly break down the internal windings, so failures are common on older motors.

Swapping a suspect coil with one from a good cylinder is a quick way to confirm the problem before ordering. Keep the coil terminals clean and tight, and route the spark plug leads away from hot or sharp surfaces. If several cylinders show the same problem at once, look at the shared wiring, the battery supply, and the ignition module before replacing every coil. Find more ignition parts in our engine controls & ignition.

• Fits Mercury 135-300 HP, 1998-2006
• Replaces Mercury 339-850227 and 339-856991A1
• Crosses to Sierra 18-5187 and CDI 184-0005
• Converts low voltage to firing voltage