P0566 means the engine control module detected a signal outside its expected range on the cruise control off circuit. Its siblings cover the other buttons: P0565 (on), P0567 (resume), P0568 (set), P0569 (coast), P0570 (accelerate), and P0575 for the switch input circuit as a whole. Which number a given fault reports depends on how the manufacturer maps its switch; on many vehicles the on and off buttons are two resistances on the same wire, and a fault reports under whichever value the module read last.
Severity: low. Can you keep driving? Yes. Cruise will not engage, or cancels on its own. The engine is otherwise normal. An airbag warning light beside this code is worth attention, because the clockspring that carries the cruise signal also carries the driver airbag circuit.
How the switch circuit works
Most cruise switches are a resistor ladder: one signal wire, one ground, a different resistor behind each button. The module feeds the signal through a pull-up and reads the voltage. Each button produces its own value; nothing pressed reads the pull-up voltage. The module sets a code when it sees a voltage that does not match any button, or a valid button held far longer than a person would hold it.
That last case is the one Audi documented: a bulletin for the A4 and A5 family with DTC P056800 (cruise control switch E45, implausible signal) describes cruise that sporadically does not work because of a jammed button. A button that sticks holds its resistance in the circuit; the module reads a button pressed for minutes and flags the switch.
Causes, in the order they are found
A sticking or contaminated button. Coffee, hand lotion and wear inside the switch make one button hold or read off-scale. A switch with one wrong button is the switch.
A failing clockspring, the coiled ribbon behind the wheel. Conductors fatigue, and the cruise signal flickers or opens with wheel position, usually with the horn, radio buttons or airbag light misbehaving too.
A chafed column harness between the clockspring and the module, at the tilt mechanism.
A connector problem after airbag or wheel work.
The module, last, after the switch reads correctly at every button.
A diagnostic order
- Record the code and companions. An airbag code points at the clockspring immediately.
- Test the rest of the wheel: horn at several wheel positions, radio buttons, heated wheel.
- Watch the cruise switch voltage live and press each button. A button that produces the wrong value or no change is the switch. A value that drifts with wheel movement and nothing pressed is the clockspring.
- Measure the switch at its connector if live data is unavailable: each button's resistance against the list in the service manual.
- Check continuity through the clockspring with the battery disconnected and the airbag precautions followed, while turning it through its range.
- Replace what measured wrong, center a new clockspring correctly, clear and verify on a road test.
Frequently asked questions
Can I drive with a P0566 code?
Yes. Cruise control will not work, and nothing else is affected. If the airbag light is also on, have the clockspring checked promptly.
What is the difference between P0565 and P0566?
P0565 is the on signal, P0566 the off signal. On most vehicles they are two resistances on the same wire, so the same failed switch or clockspring can set either, depending on which value the module read at the moment of the fault. The diagnosis is the same.
Can a stuck button set P0566?
Yes. Audi's bulletin on the A4 and A5 cruise switch describes exactly that: a jammed button holds its resistance in the circuit, the module sees a button pressed for far too long, and it stores a switch code and disables cruise.
How much does it cost to fix P0566?
A steering wheel switch is an inexpensive to moderate part with a short replacement. A clockspring is similar with about an hour of labor and airbag precautions. A column harness repair is usually under an hour.
Related codes
- P0565: cruise control on signal; see P0565.
- P0567 to P0570: resume, set, coast, accelerate.
- P0571: brake switch A circuit; see P0571.
- P0575: cruise control input circuit.
- P0578: cruise control multi-function input A stuck; see P0578.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.