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Garmin Autopilot Heading Does Not Match GPS Heading (COG)
The GPS Heading (COG) is not a reliable heading indicator at low speeds. The GPS Heading may be accurate at high speeds, but it indicates the boat's absolute motion, including the effects of winds, tides, and currents. The magnetic heading from the Garmin Autopilot only shows which direction the bow is pointing.
If the heading from the autopilot appears off, check the Reactor™ mounting location for magnetic interference with a handheld compass and ensure it is securely mounted. Check that all connected chartplotters and the GHC™ autopilot controller displays have the same North Reference (Magnetic or True), then perform the Compass Calibration and Set North procedures again.
On the GHC™ 50 autopilot controller or current model chartplotters, select the 
> Autopilot Setup > Autopilot Installation set up > Compass Setup > Compass Cal. > Begin follow the on screen directions to complete the calibration. See the Configuration Guide of the autopilot for additional considerations while performing the compass calibration.
Dealer Mode for discontinued models
From the GHC™ 20 autopilot controller
Select Menu > Setup > System > System Information > Press and hold the center soft key until Dealer Mode displays > Press the back button twice follow the steps under Calibrate Compass
Calibrate Compass
From the GHC™ 20 autopilot controller
While performing the compass calibration be sure to keep the boat as steady and flat as possible. The boat should not list during calibration. Make sure the boat is evenly loaded.
Select Dealer Autopilot Setup > Compass Setup > Calibrate Compass - Begin follow the onscreen directions to complete the calibration.
Set North
From the GHC™ 20 autopilot controller
When performing the set North Procedure, you must have a large stretch of open water available, calm surface conditions are best for setting North. Begin driving your boat in a straight line at cruising speed, downwind, and in the same direction as the current.
Select Dealer Autopilot Setup > Compass Setup > Set North – Begin Continue to drive the boat in a straight line at cruising speed, downwind in the same direction as the current and follow the on-screen instructions.