Why does my marine autopilot drive in circles instead of following waypoints?

An autopilot that drives in circles has usually lost its heading data, meaning the course computer is blindly chasing a compass bearing it can never reach. The culprit is typically a malfunctioning heading sensor, severe magnetic interference, or a broken rudder reference unit feeding false angles back to the brain.

Check the Heading Sensor

Your autopilot relies on a heading sensor to know where the bow points. Older systems use a mechanical fluxgate compass (like the classic Raymarine or Autohelm black dome). Modern setups use solid-state 9-axis sensors like the Raymarine EV-1, Garmin Reactor 40, or B&G Precision-9. If this sensor fails or becomes disorientated, the autopilot continuously turns the rudder to find a heading that does not exist.

Before checking the wiring, look for magnetic interference. Owners frequently stow steel toolboxes, handheld VHFs, or magnetic torches in lockers right next to the hidden compass. Remove any ferrous metal within a metre of the sensor. Next, look at your autopilot control head (such as a Raymarine p70s or Garmin GHC 20) and check the current heading. Compare it to your conventional steering compass or your GPS Course Over Ground (COG) while underway. If the actual heading is 180 degrees but the autopilot reads 090 degrees, you have found the problem. You may need to perform a factory reset and complete a sea trial to swing the compass and recalibrate.

You can test older Autohelm or Raymarine fluxgate compasses with a multimeter. Disconnect the compass from the course computer and measure the resistance across the pins. You should see around 8 ohms between the red and green wires, and between the red and yellow wires. An open circuit means the fine internal wiring has snapped. You need a new sensor.

Inspect the Rudder Reference Unit

If the heading data is perfectly accurate but the boat still turns in circles, suspect the Rudder Reference Unit (RRU). This small sensor sits on the rudder quadrant and tells the course computer exactly where the rudder is positioned. If the internal potentiometer burns out or suffers water ingress, the computer might think the rudder is dead centre when it is actually hard over to port.

Take a torch into the lazarette and physically inspect the RRU. Keep clear of the steering quadrant while someone turns the wheel to avoid trapping your fingers.

• Check the mechanical linkage. The small threaded rod and plastic ball joints frequently pop off or snap.
• Ensure the sensor arm moves smoothly in tandem with the steering quadrant.
• If the linkage is intact, check the rudder angle bar on your autopilot display. If you turn the wheel hard to starboard but the display shows the rudder amidships or flickers wildly, the RRU needs replacing. A new Raymarine rotary RRU costs around £250.