Why would a fault in the off signal stop cruise control from turning on?

Because the module treats the ability to cancel as a precondition for engaging. Cruise control opens the throttle without the driver's foot on the pedal, and the safety case for that depends entirely on being able to stop it instantly. If the module cannot verify that its cancel path is healthy, most control strategies decline to arm the system at all. So the complaint is that cruise will not switch on, and the actual fault is that it cannot be reliably switched off.

Could my brake light switch really be the cause?

It is the most likely single cause. The brake pedal switch is a cancel path with the same authority as the cruise cancel button, it is operated hundreds of times a day, and it holds its adjustment against a moving pedal arm. When it drifts or wears, the module gets an unreliable brake signal and this code follows. Look at your brake lights: if they stay on with the pedal released or need a hard push to come on, you have found it. A $20 switch and a few minutes of adjustment fixes a surprising proportion of these.

I drive a manual and my brake lights are fine. What else should I check?

The clutch pedal switch. On a manual transmission it is a full cancel path in exactly the same way as the brake switch, and it is adjusted in the same style, but because nothing on the outside of the car depends on it nobody notices when it drifts out of position or wears out. It is an inexpensive part, it lives at the top of the clutch pedal arm, and checking it takes minutes. Rule it out before anyone removes a steering wheel.

Is it dangerous to drive with this code stored?

The vehicle itself is safe to drive and the engine is unaffected. In most cases cruise control will simply refuse to engage, which removes the risk rather than creating it. The situation that warrants care is the rare one where cruise still engages but the cancel button does not stop it — if that is happening, the brake pedal will still disengage the system on every vehicle, but you should have it repaired promptly rather than relying on that, and leave the system switched off in the meantime.

OBD-II trouble code

P0566: Cruise Control "Off" Signal

The cancel input is the one cruise circuit the module treats as a safety function, so a fault here often disables the whole system on purpose. It is also the code most likely to be caused by something that is not a cruise part at all — the brake pedal switch.

Low severityPowertrainCruise ControlDrivable short-term

Quick facts

System: Powertrain
Category: Cruise Control
Severity: Low severity
Drivable: Usually safe to drive short-term
Repair cost range: $0 – $400
DIY difficulty: Intermediate DIY

What does P0566 mean?

P0566 covers the signal that tells cruise control to stop — the off or cancel input. It is easy to read that as the least important code in the family, since a failure to cancel sounds like a minor inconvenience next to a failure to set. The reality is the opposite, and understanding why explains almost everything about how this code behaves.

Cruise control is a system that commands the throttle without the driver's foot on the pedal. The entire safety case for allowing that rests on being able to stop it immediately and reliably. Control strategies therefore treat the cancel path as a permission rather than a feature: the module must be satisfied that it can be commanded off before it will allow itself to be switched on. When this input is faulted, a great many vehicles refuse to arm cruise control at all. The driver's complaint is that cruise will not engage, and the cause is a broken ability to disengage — which is not an obvious chain of reasoning if you do not know the design intent.

The second, and more useful, thing to know is that cancel is not a single wire from a single switch. Every vehicle has multiple independent ways to stop cruise control, and they are deliberately redundant. Pressing the cancel button is one. Touching the brake pedal is another. On a manual transmission, depressing the clutch is a third. Shifting out of drive is often a fourth. Because these paths converge on the same function, a fault in any of them can land in this code — and the brake pedal switch is by a wide margin the most likely culprit, because it is a cheap mechanical part that is actuated hundreds of times a day and holds its adjustment against a pedal arm that moves.

That gives this code a diagnostic shortcut nothing else in the cruise family has: look at the brake lights. A brake switch that is out of adjustment, has drifted on its threads, or has worn internally will usually show itself there — lights staying on with the pedal at rest, or requiring an unusually firm press to illuminate. Either behaviour means the module is receiving an unreliable brake signal, and an unreliable brake signal is an unreliable cancel path. Repairing it frequently resolves this code with no cruise component touched at all. If a brake switch code is stored alongside this one, that conclusion is close to certain.

The clutch switch deserves a specific mention because it is so easily missed. On a manual car it has the same cancel authority as the brake switch and it is adjusted in the same way, but nothing externally visible depends on it, so nobody notices when it drifts. A manual vehicle with this code and perfectly normal brake lights should have its clutch switch checked before the steering wheel is taken apart.

Common causes

Maladjusted, drifted or failed brake pedal switch, which is a cancel path in its own right
Worn clutch pedal switch on manual transmission vehicles
Worn contacts in the cancel or off position of the cruise switch assembly
Short to ground on the cancel circuit, holding the module at a permanent stop request
Open circuit in the cancel wiring between the switch and the module
Corroded connector at the brake pedal bracket, where moisture from wet footwells collects
Aftermarket alarm, remote start or trailer wiring spliced into the brake light circuit
Damaged clockspring ribbon where the cancel button is on the steering wheel

Symptoms

Cruise control refusing to engage at all, despite every button appearing to work
Cruise engaging and then cancelling itself after a few seconds
Brake lights staying on with the pedal released, or not illuminating until the pedal is pressed hard
A brake switch fault stored alongside this code
Cruise failing to cancel when the cancel button is pressed, requiring the brake to disengage it
Difficulty shifting out of park on vehicles with a brake-interlocked shifter
Complaint appearing after alarm, remote start or trailer wiring installation
Check engine light with no change in how the engine runs

Diagnostic steps

1.Check the brake lights before you touch anything cruise-related. On the great majority of vehicles the brake pedal switch is a cancel path in its own right, and a maladjusted or failed one both disables cruise control and sets faults in this area. Brake lights that stay on with the pedal released, or that need a firm push to come on, have told you where to go.
2.Scan for brake switch codes alongside this one. This code arriving together with a brake switch or brake switch correlation fault is not two problems — it is one, and it is almost never the cruise stalk. Fix the brake switch, clear both, and retest before considering anything else.
3.Verify brake pedal switch adjustment physically. Many are threaded or ratcheting units that set their own position against the pedal arm on installation, and they drift. There should be a small amount of pedal free play before the switch changes state, and the plunger must fully extend when the pedal is released.
4.Read the cancel input's live state with the key on and nothing touched. It should show inactive. An input showing cancel asserted at rest means the module is permanently being told to shut the system down, which is why cruise will not engage even though every other button tests fine.
5.Operate the dedicated Cancel or Off control while watching live data, if the vehicle has one separate from the main switch. You want a clean momentary transition. A slow return or a bouncing signal indicates worn contacts and is the point at which the switch assembly is the answer.
6.Check the clutch switch on manual transmission vehicles. It is a second cancel path with exactly the same authority as the brake switch, it is adjusted the same way, and it is forgotten far more often because nothing visible on the outside of the car depends on it.

Repair cost

$0 – $400

The floor is zero because a large share of these are a brake pedal switch that has drifted out of adjustment, and readjusting it takes a few minutes and no parts. A brake light switch itself is $15 to $60 and is usually a fifteen-minute job under the dash, making it one of the cheapest genuine fixes in the whole OBD library. A clutch pedal switch is similar. Diagnosis is $80 to $150. If the fault turns out to be in the cruise switch assembly rather than a pedal switch, expect $60 to $350 fitted depending on whether the cancel button is a separate part or built into a larger multi-function unit. Check the pedal switches first — the cost difference between the two outcomes is large and the test is free.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.