Autopilot Follow-Up Error: What It Means

What the follow-up error reading is

On many autopilot control heads, the status page includes a reading labeled follow-up error, sometimes shown as FU error. It belongs to follow-up (FU) mode, the autopilot mode where you steer through the control head rather than through the boat's helm. In that mode, you command a rudder angle with a button or knob, and the autopilot moves the drive until the rudder reaches the commanded angle. The follow-up error is the difference between the commanded angle and the actual rudder angle reported by the rudder reference.

The value is a setup diagnostic, not a performance rating. Its job is to tell you whether the rudder is actually reaching the angle you commanded. When the value settles near zero, the autopilot and the steering are responding as expected. When it stays large or grows, something in the loop is preventing the rudder from following the command.

How follow-up mode works on a control head

Follow-up mode is described in the maker's manual for each control head. The control head sends a target rudder angle to the course computer or actuator control unit, the drive moves the rudder, and the rudder reference reports back the actual angle. The difference between the two is what the control head displays as the follow-up error. On some systems the reading appears only during setup; on others you can call it up while steering. The manual for your model tells you which page shows it.

The same parts that matter for this reading are the parts you compare when choosing an autopilot: a control head and a rudder reference. A pack that includes both gives you the full loop. A core pack without a control head cannot show you the error reading until you add one.

What the reading tells you during setup

During commissioning, the maker's manual typically asks you to run the drive through its full travel and watch the follow-up error. You command rudder to one side, let it reach the stop, and note the reading. A small value means the commanded angle and the reported angle agree. A large value means the rudder reference is not reporting the same angle the drive is producing, or the drive is not moving the rudder as far as the command asks.

The likely causes are listed in the maker's manual: a rudder reference that is not calibrated, a feedback lever that is not adjusted to the steering arm, a drive that is not sized for the steering system, or air in a hydraulic system. Do not adjust any of these from a website. The manual's commissioning steps, described in the calibration article, and a qualified installer are the correct path.

Why the rudder reference sets the number

The follow-up error is only as accurate as the rudder reference. A rudder reference unit, also called rudder feedback, measures the actual rudder angle and sends it to the autopilot. If that unit is not installed, not connected, or not calibrated, the autopilot has no way to know where the rudder is, and the follow-up error reading becomes meaningless.

That is why a listing's stated details matter. Some complete autopilot packs name a rudder reference in the box; some core packs leave it out. When you are comparing autopilots on stated details, the presence of a rudder reference is a detail you can check. The behavior of a particular unit under way is something neither the listing nor this site can tell you.

What a growing error under way suggests

If you switch to follow-up mode while the boat is moving and the error grows, the rudder is not reaching the commanded angle. The usual suspects are the same as during setup: a slipping clutch, air in a hydraulic steering system, a mechanical obstruction in the steering linkage, or a drive that is not matched to the steering system.

This article will not tell you how to repair any of these. The drive, the clutch, the hydraulic system, and the rudder reference are all covered in the maker's manual, and the right hand to work on them is a qualified installer's. For the general direction, see the troubleshooting basics. Your job before a purchase is to know your steering type and to pick a pack whose stated details match it.

How the reading should shape your listing choices

A follow-up error is a diagnostic value, so it never appears in an autopilot listing. What you can read in a listing is the equipment that determines whether the reading will make sense: whether a rudder reference is named, what drive type is in the box, whether the pack has a control head, and which steering type the pack states. A listing that does not mention a rudder reference is not a bad listing, but it is a reason to ask the seller what the pack includes.

The site's buying guides compare autopilots on stated details from the maker's pages. That comparison covers drives, control heads, rudder references, and steering types, the same parts that decide how the follow-up error reading behaves during setup. Use those guides, like the control head comparison and the hydraulic autopilot comparison, to build a shortlist, then read the maker's manual for the commissioning steps.

What to pick for your situation

Questions

Is the follow-up error reading the same on every autopilot?

No. Each maker uses its own status pages, labels, and setup procedures. The general meaning is the same, the difference between commanded and actual rudder angle, but the exact name of the reading and where it appears come from the manual for your model.

Can the follow-up error reading be used to compare two autopilots?

No. It is a setup diagnostic, not a published spec. Listings do not state it, and no number in this article or on this site claims to be a measured value. Compare the parts instead: rudder reference, drive type, and control head.

What should I do if the reading stays large after I follow the manual?

Stop. The commissioning steps in the manual are the limit of what an owner should adjust. If the reading still stays large, a qualified installer or the maker's dealer should look at the rudder reference, the drive, and the steering system.

Why does the reading only matter during setup and not on a listing?

Because setup is the moment the autopilot and the steering are first allowed to move together. A listing is a statement of what is in the box, not a promise about how the parts will behave on your boat.

Does the follow-up error affect the autopilot's normal course-keeping?

In general terms, the follow-up error belongs to follow-up mode, not to normal automatic steering. A large setup error can point to faults in the rudder reference or drive that would also matter in automatic mode, but the value itself is read in follow-up mode.