Reciprocating aircraft engines are equipped with magnetos that provide electric current to the spark plugs. The magnetos are independent from the electrical system and will continue to provide current to the spark plugs even after a complete electrical failure. When the magneto switch is closed (OFF), a path to ground exists and the magneto will not spark the plug. If the magneto switch is open, it is in the ON position, enabling the magneto. If the P-Lead should become discontinuous (broken), the magneto switch cannot disable the magneto, so the magneto can fire the plugs even though the magneto switch is OFF. This is why a magneto switch check is done at idle RPM to determine that the magneto switch is indeed controlling the magneto. In the BOTH position, all of the switches are open. This allows both magnetos to be operational. In the LEFT position, all of the switches are open except the one which grounds the right magneto. In the RIGHT position, all of the switches are open except the one which grounds the left magneto. In the OFF position, both magnetos are grounded, and the switches to the starter solenoid and vibrator and those in the retard circuit are open.

Preflight Magneto Check: If the engine runs rough on either magneto—or if the RPMs drop below the value specified in the POH, the spark plugs could be contaminated. A small bit of lead or carbon deposit can block the air gap that electricity must jump to create a spark. To fix this problem, set the ignition on both mags, run the engine up to full cruise of about 2300 RPM, then lean the mixture to best power for about a minute. This procedure will create higher temperatures within the cylinders that may burn off the contamination and restore a proper spark. Reduce power and try the normal magneto check again. If engine still doesn't run smoothly, run up the engine again and watch the EGTs on the engine analyzer. They should rise in sequence when one mag is selected. If not, maintenance is needed—there could be a spark plug or ignition harness problem, or a mis-timed magneto.

If there is no RPM drop during the magneto check, a broken P-lead may be preventing the switch from disabling the magneto. The flight should be cancelled and the engine should be checked by a mechanic.