[CA] [CA11] St8001 display stuck on 15 degrees

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I have a 30 ft crab boat equipped an st8001 auto pilot system. The display indicates the rudder is at 15 degrees starboard no matter the actual rudder position. If I engage the auto pilot to dead reacon, it will not operate properly because it is reading the false rudder angle. In standby made, I have complete control of the wheel. However, the display will occasionally show cross track error.
I have checked all connections and terminations. I followed the trouble shooting procedure for the rudder reference indicator as described in download from raymarine. All ohm readings indicated the rudder reference is working properly.
What is my next step of troubleshooting so I don't have to blindly waste money on replacing parts. Is there a test for the dash mounted display and the "brain" that is mounted in the cuddy cabin? Thx Vince

[Chuck - Raymarine - Moderator]

Welcome to the Raymarine Forum pacificlegend1,

Unfortunately, should the rudder reference transducer test not indicate a problem with the transducer, then the fault would be determined to be within the autopilot's course computer's circuitry. Should the autopilot be an SPX5 , SPX10, or SPX30, then it would be recommended that the autopilot course computer be sent to Raymarine’s Product Repair Center to be bench checked / serviced. Unfortunately, should the autopilot course computer not be a SPX5 , SPX10, or SPX30, then the autopilot course computer would need to be replaced with a second hand Raymarine autopilot course computer of the same type or with an Evolution autopilot (the currently installed drive unit and rudder reference transducer may be retained for use with the Evolution autopilot).