Why won't my Garmin autopilot keep the boat on course?

Your Garmin autopilot may be holding a compass heading while wind and tide push the boat off the plotted track, or there may be a fault in its heading sensor, steering drive or setup. Check the operating mode first, then compare the displayed heading with the steering compass and watch whether the rudder responds to small course changes.

Check the steering mode

The Garmin screen is the control interface; the heading sensor, autopilot computer and drive do the steering. A working display does not prove the whole system works.

Start by distinguishing between three faults:

• The heading stays steady, but the boat leaves the chart line: heading hold can be working correctly. It maintains the direction the bow points, allowing tide and leeway to carry the boat sideways.
• The heading swings repeatedly either side of the target: suspect steering play, unsuitable tuning, poor heading information or insufficient steering authority.
• The boat steadily turns away from the target: check whether the drive operates, whether steering direction is correct and whether the heading reading follows the turn.

To follow a route, the pilot normally needs an active Go To or route on the chartplotter, with the appropriate GPS steering mode engaged. Displaying a route alone does not make the autopilot follow it. Names and engagement steps vary between GHC controllers, GPSMAP displays and autopilot generations. Wind hold on a sailing installation is a separate mode.

Carry out steering tests in clear water at a safe speed, with someone at the helm ready to select Standby and take manual control.

Check the heading data

Display heading, rather than GPS course over ground, and compare it with a trustworthy steering compass on several steady headings. Both readings must use the same magnetic or true reference. Course over ground describes movement across the seabed and can differ from heading in a cross-tide. At very low speed, it is a poor compass check.

A consistent offset suggests heading alignment or reference settings. An error that changes substantially as the boat turns suggests magnetic interference or a calibration problem. Sudden jumps suggest an intermittent sensor, connection or power issue.

• Check for tools, speakers, spare anchors or recently fitted electrical equipment near the heading sensor or course computer unit, often called the CCU.
• Inspect accessible network plugs for loose locking collars, moisture and damaged pins.
• Check that the NMEA 2000 network is powered and the expected heading source appears in the device list.

Correct interference and connection problems before recalibrating. Garmin compass calibration and heading alignment procedures depend on the installed system. Use its commissioning instructions rather than another manufacturer's menu sequence.