P0591 Cruise Control Multi-Function Input "B" Circuit Range/Performance

P0591 is an OBD-II diagnostic trouble code that indicates a range or performance problem in the Cruise Control Multi-Function Input "B" circuit.

In simple terms, the vehicle's engine or body control module is receiving a signal from the cruise-control switches, steering-wheel controls, or related multi-function input circuit, but the signal is not within the expected electrical range or does not behave as the control module expects.

This does not necessarily mean that the cruise-control system itself has completely failed. The problem can be caused by the switch assembly, steering-wheel wiring, clock spring, connector, reference voltage, ground, or an abnormal input signal.

What Does P0591 Mean?

The code can be broken down into several parts:

• P – Powertrain-related diagnostic code

• 05 – Cruise control and related speed-control system

• 91 – Specific fault identifier

• Cruise Control Multi-Function Input "B" – A designated input circuit associated with the cruise-control or multifunction switch system

• Range/Performance – The module sees a signal, but the signal is outside the expected range or does not change correctly

The important point is that "Range/Performance" does not automatically mean that the circuit is open or shorted.

The circuit may still be electrically connected, but the voltage or signal being returned to the control module may be incorrect.

How Does the Cruise Control Multi-Function Input Work?

Modern cruise-control systems can use several buttons or switches, often mounted on the steering wheel or steering column.

Typical functions may include:

• Cruise control ON/OFF

• SET

• SET-

• RES

• RES+

• CANCEL

• Speed limiter

• Distance or driver-assistance functions on some vehicles

• Other multifunction buttons

Depending on the vehicle, these buttons may communicate with the ECU/PCM through a dedicated circuit, a resistor network, a body control module, or a digital communication network.

In some systems, several buttons share a single signal wire.

For example, the control module may supply a reference voltage to the switch assembly. Pressing different buttons introduces different resistance values into the circuit. The module then interprets the resulting voltage as a particular command.

This means that a small change in resistance, a poor connector, corrosion, or a damaged steering-wheel wire can cause the module to interpret a button as the wrong command.

What Does "B Circuit" Mean?

The "B" designation does not necessarily mean a specific physical wire that is universally called Circuit B.

It is a manufacturer-defined designation.

One vehicle may use Circuit B for a particular steering-wheel switch input, while another vehicle may use a different circuit architecture.

For this reason, the exact location and wiring of the P0591 circuit should always be confirmed using the vehicle's wiring diagram and service information.

What Does "Range/Performance" Mean?

This part of the code is particularly important.

The control module expects to see certain electrical values when different cruise-control buttons are pressed.

For example, a simplified system might expect:

• No button pressed → approximately one voltage

• SET button → another voltage

• RES button → another voltage

• CANCEL → another voltage

• ON/OFF → another voltage

The actual values vary considerably between manufacturers.

If the module receives a voltage that is technically possible but does not correspond to the expected range, it can set P0591.

The same can happen if the signal changes incorrectly when a button is pressed.

Symptoms of P0591

The most common symptom is a problem with the cruise-control system.

Possible symptoms include:

• Cruise control does not operate

• Cruise control cannot be switched on

• SET function does not work

• RESUME function does not work

• CANCEL function behaves incorrectly

• Certain steering-wheel buttons do not work

• Cruise-control indicator does not appear

• Speed limiter functions incorrectly

• Cruise control works intermittently

• Check Engine Light comes on

• Other steering-wheel control functions may stop working

In some vehicles, the driver may notice no obvious drivability problem at all.

The vehicle can continue to accelerate, idle and drive normally because P0591 generally concerns the control input used to operate the cruise-control system rather than the basic engine-control system.

Can P0591 Cause the Cruise Control to Stop Working?

Yes.

This is one of the most likely consequences of the code.

If the ECU cannot reliably determine what the driver is requesting through the multifunction input circuit, it may disable cruise control as a safety measure.

For example, if the module cannot distinguish between SET, RESUME and CANCEL commands, it cannot safely operate the system.

Therefore, the cruise-control system may be disabled until the electrical problem is corrected.

Common Causes of P0591

Several problems can cause this code.

1. Faulty cruise-control switch

The multifunction switch itself may have an internal electrical problem.

Contacts can wear out, become contaminated or develop excessive resistance.

A switch may also work intermittently, making the problem more difficult to identify.

2. Damaged steering-wheel wiring

The wiring between the steering-wheel controls and the rest of the vehicle can become damaged.

This is especially important on vehicles where the wires pass through the steering-wheel area.

Repeated steering-wheel movement can contribute to wiring fatigue.

3. Faulty clock spring

On vehicles with steering-wheel-mounted cruise-control buttons, the signals must pass between the stationary steering column and the rotating steering wheel.

The clock spring provides this electrical connection.

A damaged clock spring can therefore affect:

• Cruise-control buttons

• Horn

• Steering-wheel controls

• Airbag circuits

If several steering-wheel functions stop working simultaneously, the clock spring becomes an important area to investigate.

4. Corroded connector

Moisture or corrosion at a connector can alter the electrical resistance of the circuit.

This may cause the control module to see an incorrect voltage.

5. Loose connector

A connector that is not fully seated can cause intermittent electrical contact.

The cruise control may work sometimes and fail at other times.

6. High resistance in the circuit

A partially damaged wire or poor terminal connection can introduce resistance into the circuit.

This can change the voltage seen by the control module.

7. Short circuit

A signal wire may be contacting:

• Ground

• Battery voltage

• Another circuit

The exact effect depends on the circuit design.

8. Reference voltage problem

Some cruise-control input circuits use a reference voltage.

If that voltage is incorrect, the module may interpret the switch signals incorrectly.

9. Ground problem

A poor ground can also change the electrical characteristics of the circuit.

Ground problems should therefore not be overlooked.

10. Steering-wheel switch contamination

Liquid, dirt or other contamination around the buttons can sometimes affect switch resistance or cause abnormal electrical readings.

11. Body control module or communication problem

On some vehicles, the steering-wheel buttons do not communicate directly with the engine ECU.

The signal may pass through a body control module or another control unit.

A communication or module problem can therefore produce symptoms that look like a switch failure.

12. Control module fault

An ECM, PCM, BCM or other module can theoretically be responsible.

However, the control module should generally be considered after the wiring, power supply, ground, connectors and switch circuit have been properly tested.

Can a Weak Battery Cause P0591?

A weak battery or unstable vehicle voltage can sometimes cause unusual electronic faults.

However, a low battery should not automatically be considered the primary cause of P0591.

If the vehicle has recently experienced:

• Battery replacement

• Very low battery voltage

• Jump starting

• Charging-system problems

• Multiple unrelated electronic codes

then battery and charging voltage should be checked.

If P0591 repeatedly returns under normal voltage conditions, the cruise-control input circuit itself deserves closer investigation.

Is P0591 Serious?

P0591 is generally not considered an immediate engine-damage code.

In many vehicles, the primary consequence is the loss or malfunction of cruise-control functionality.

However, it should still be diagnosed because an abnormal control input can cause unpredictable operation of the cruise-control system.

Until the problem is repaired, it is sensible to avoid relying on cruise control.

Can You Drive With P0591?

Usually, yes.

If the engine runs normally and the only symptom is that cruise control does not operate, the vehicle can often be driven normally.

However, the situation is different if P0591 appears together with other electrical or steering-wheel faults.

Pay particular attention if you also have:

• Airbag warning light

• Horn failure

• Multiple steering-wheel controls not working

• ABS/ESC warnings

• Communication-related fault codes

• Sudden electrical failures

In those circumstances, the underlying electrical problem may be more significant than P0591 itself.

How Is P0591 Diagnosed?

A professional diagnosis should begin with the simplest possibilities and progress toward more detailed electrical testing.

1. Scan all control modules

Start with a diagnostic scanner capable of reading more than just generic engine codes.

Record:

• P0591

• Other stored codes

• Pending codes

• Freeze-frame information

• Cruise-control-related data, if available

Other codes can provide important clues.

For example, a communication code combined with P0591 may point toward a module or network problem rather than a simple switch failure.

2. Check whether the cruise-control buttons work

Test every relevant button.

Check:

• ON/OFF

• SET

• RES

• CANCEL

• Speed adjustment

• Limiter functions, if equipped

If only one button is affected, the switch assembly or its individual circuit becomes more suspicious.

If every steering-wheel button is dead, the problem may be farther upstream.

3. Check the horn and other steering-wheel functions

This is particularly useful on vehicles using a clock spring.

If the cruise-control buttons, horn and other steering-wheel functions fail at the same time, inspect the clock spring and steering-wheel wiring carefully.

4. Inspect the connectors

Look for:

• Bent terminals

• Loose terminals

• Corrosion

• Water contamination

• Damaged locking mechanisms

• Pulled-out wires

Electrical problems frequently originate from connectors rather than expensive electronic modules.

5. Inspect wiring around the steering column

Look for:

• Pinched wires

• Broken wires

• Chafing

• Previous repair work

• Incorrect aftermarket wiring

• Damaged insulation

Do not simply look at the wiring visually and assume it is good.

A wire can look normal externally while being broken internally.

6. Monitor the input signal

If the scan tool provides live data for the cruise-control switch input, observe the parameter while pressing each button.

The value should change consistently and correspond to the expected button.

If the signal jumps between values or remains at an unexpected value, further electrical testing is required.

7. Measure the circuit

Using the wiring diagram and manufacturer specifications, measure the relevant voltage, resistance and continuity.

Do not apply generic voltage or resistance values from another vehicle.

The correct values depend on the system design.

8. Check the clock spring

If the circuit passes through a clock spring, its continuity should be tested according to the manufacturer's procedure.

The steering wheel and airbag system require appropriate safety procedures when accessing these components.

9. Check power and ground

Verify the relevant fuses, power supplies and grounds.

A poor ground can produce unusual input readings and lead to incorrect diagnosis.

10. Test the switch itself

If the wiring and power supplies are correct, test the multifunction switch.

Compare its resistance or signal output with the manufacturer's specifications.

Why a Multimeter Is Important for P0591

A diagnostic scanner tells you what the control module believes is happening.

A multimeter can help determine why the module is receiving that information.

For example, suppose the scanner shows that the cruise-control input is constantly reading an unexpected state.

That does not automatically prove the ECU is faulty.

The actual cause could be:

• Incorrect reference voltage

• High resistance

• Short to ground

• Short to voltage

• Faulty switch

• Damaged clock spring

• Poor connector contact

Electrical testing helps separate these possibilities.

A Simple Diagnostic Example

Imagine a vehicle has several cruise-control buttons sharing one input circuit.

The control module expects different voltage values depending on which button is pressed.

The driver presses SET, but the voltage is much higher than expected.

The module cannot identify the command correctly and eventually stores P0591.

Replacing the ECU would be a poor first step.

A better diagnostic sequence would be:

Switch → connector → clock spring → wiring → reference voltage → ground → control module

This approach prevents unnecessary replacement of expensive components.

P0591 and the Clock Spring

The clock spring deserves special attention because it is physically subjected to repeated rotation.

If P0591 occurs together with:

• Non-functioning steering-wheel buttons

• Horn failure

• Airbag warning

• Steering-wheel control problems

the clock spring should be investigated.

However, an airbag warning should not be ignored merely because P0591 is present. Airbag circuits require their own diagnosis and appropriate safety procedures.

P0591 and Aftermarket Equipment

Aftermarket accessories can occasionally contribute to cruise-control input problems.

Examples include:

• Aftermarket steering wheels

• Remote-control installations

• Alarm systems

• Audio-control modifications

• Incorrectly installed multimedia systems

• Non-original cruise-control switches

If the problem appeared immediately after electrical modifications, inspect that work before replacing factory components.

P0591 vs P0592

These codes are closely related.

P0591 – Cruise Control Multi-Function Input "B" Circuit Range/Performance

The control module detects that the input signal is present but outside the expected range or does not perform correctly.

P0592 – Cruise Control Multi-Function Input "B" Circuit Low

The control module detects a low electrical signal on the same designated input circuit.

Therefore, P0591 generally points toward a signal plausibility or range problem, while P0592 specifically identifies a low signal condition.

The exact electrical interpretation still depends on the vehicle manufacturer.

P0591 vs P0593

P0593 – Cruise Control Multi-Function Input "B" Circuit High

This indicates that the module detects a high signal condition on the designated input circuit.

In simplified terms:

• P0591 → signal range/performance problem

• P0592 → signal too low

• P0593 → signal too high

These three codes can be useful together when diagnosing the electrical behavior of the circuit.

Other Related Cruise-Control Codes

Depending on the vehicle, you may encounter related codes such as:

• P0564 – Cruise Control Multi-Function Input "A" Circuit

• P0565 – Cruise Control On Signal

• P0566 – Cruise Control Off Signal

• P0567 – Cruise Control Resume Signal

• P0568 – Cruise Control Set Signal

• P0569 – Cruise Control Coast Signal

• P0570 – Cruise Control Accel Signal

• P0571 – Brake Switch "A" Circuit

• P0590 – Cruise Control Multi-Function Input "B" Circuit

• P0591 – Cruise Control Multi-Function Input "B" Circuit Range/Performance

• P0592 – Cruise Control Multi-Function Input "B" Circuit Low

• P0593 – Cruise Control Multi-Function Input "B" Circuit High

If several of these codes are stored simultaneously, the diagnosis should focus on the common circuit, power supply, wiring or control module rather than immediately replacing individual switches.

Common Diagnostic Mistakes

One of the biggest mistakes with P0591 is replacing the cruise-control switch without testing the circuit.

A faulty switch is certainly possible, but it is not the only explanation.

Another common mistake is replacing the ECU simply because the scanner identifies an input problem.

The ECU may be correctly detecting an abnormal signal generated somewhere else in the circuit.

Other mistakes include:

• Ignoring the clock spring

• Not checking connectors

• Not checking grounds

• Failing to inspect wiring

• Clearing the code without diagnosing it

• Testing resistance without following the manufacturer's procedure

• Using generic voltage specifications

• Ignoring other stored codes

• Assuming every vehicle uses the same cruise-control circuit design

How Do You Fix P0591?

The correct repair depends on the actual cause.

Possible repairs include:

• Replacing the faulty cruise-control switch

• Repairing damaged wiring

• Repairing or replacing a damaged connector

• Cleaning corrosion from appropriate electrical connections

• Repairing a poor ground

• Repairing a reference-voltage problem

• Replacing a defective clock spring

• Correcting an aftermarket wiring installation

• Repairing a body-control or communication problem

• Reprogramming or replacing a control module when properly diagnosed

The important point is that P0591 identifies a circuit problem, not automatically a failed component.

Can You Clear P0591 With a Scanner?

Yes.

A diagnostic scanner can normally erase the stored code.

However, clearing the code does not repair the underlying problem.

If the abnormal input signal remains, P0591 will usually return after the vehicle performs the necessary diagnostic checks.

A proper repair should therefore be followed by:

1. Clear the code.

2. Start the vehicle.

3. Check the cruise-control input data.

4. Test the relevant switches.

5. Road-test the vehicle when safe.

6. Rescan the vehicle.

7. Confirm that P0591 does not return.

Final Thoughts on P0591

P0591 – Cruise Control Multi-Function Input "B" Circuit Range/Performance indicates that the control module is not receiving the cruise-control multifunction input it expects.

The problem may be inside the switch, but it can also originate from the clock spring, steering-wheel wiring, connectors, reference voltage, ground, body-control module or communication system.

In many cases, the vehicle remains completely drivable because the fault primarily affects cruise-control operation. Nevertheless, the system should be diagnosed rather than ignored, particularly if other steering-wheel or electrical functions are also malfunctioning.

The most effective approach is to treat P0591 as an electrical signal diagnosis rather than simply a "bad cruise-control switch" code. Checking the input signal, wiring, connectors and related systems before replacing expensive components can save considerable time and money.