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

P0593 – Cruise Control Multi-Function Input "B" Circuit High indicates that the engine or cruise-control module is detecting a voltage or electrical signal that is higher than expected on the cruise-control multifunction input “B” circuit.

This code is different from a cruise-control actuator or servo fault. Here, the focus is on the input signal coming from the cruise-control controls, usually the switches or steering-wheel controls used to operate functions such as SET, RESUME, CANCEL, or cruise activation.

The exact circuit design varies between manufacturers. On some vehicles, several cruise-control buttons share one or more signal circuits using different resistance values. On others, the controls communicate digitally through a steering-wheel module or CAN network. Therefore, P0593 should always be interpreted together with the vehicle's wiring diagram and control strategy.

What Does “Multi-Function Input B” Mean?

The cruise-control system needs information from the driver.

When you press a cruise-control button, the control module has to determine which button was pressed.

In a simple system, the switches may be connected through a dedicated input circuit. Different buttons can produce different voltage levels.

For example, the control module might see:

• One voltage for SET

• Another voltage for RESUME

• Another voltage for CANCEL

• Another voltage for cruise ON/OFF

The module compares the measured voltage against expected ranges.

If the voltage on Input B is higher than the programmed range, P0593 may be stored.

The exact voltage values are manufacturer-specific, so there is no universal voltage that applies to every vehicle.

Why Would the Input Voltage Be Too High?

A high input does not necessarily mean that the switch itself is defective.

The module is essentially saying:

“The electrical signal I am receiving on this input is higher than I expect.”

Possible causes include:

• Open circuit in the signal wiring

• Signal wire disconnected

• Short to battery voltage

• Damaged cruise-control switch

• Incorrect switch resistance

• Damaged steering-wheel switch assembly

• Loose connector

• Corroded connector terminals

• Damaged wiring inside the steering wheel

• Faulty clock spring

• Incorrect replacement steering-wheel control

• Wiring connected to the wrong circuit

• Control-module fault

• Communication or steering-wheel module problem on systems using networked controls

The correct diagnosis depends heavily on how the manufacturer designed Input B.

How Cruise-Control Switches Can Produce a High Signal

Many conventional cruise-control systems use a resistor ladder.

Instead of giving every button its own separate wire, several switches can share an input.

The control module sends a reference voltage through the circuit. Pressing different buttons changes the resistance and therefore changes the voltage returning to the module.

Conceptually:

Reference Voltage → Resistor/Switch Network → Input B → Control Module

If the circuit becomes open, the module may see a voltage close to the reference voltage.

That can be interpreted as a high input.

This is why an open wire can sometimes produce a code described as “Circuit High.” The problem does not necessarily mean that somebody applied excessive voltage to the circuit.

An Open Circuit Can Look Like a High Signal

This is one of the most important details when diagnosing P0593.

Suppose the control module normally expects a lower voltage when a particular switch circuit is connected.

If the signal wire breaks:

Switch → X Broken Wire X → Control Module

The input may be pulled toward the reference voltage.

The module then sees a voltage that is too high and sets P0593.

Therefore:

P0593 does not automatically mean a short to battery voltage.

An open circuit, depending on the circuit design, can produce exactly the same diagnostic result.

Symptoms of P0593

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

The vehicle may exhibit:

• Cruise control not activating

• SET function not working

• RESUME not working

• CANCEL behaving incorrectly

• Cruise-control buttons becoming unresponsive

• Cruise control working intermittently

• Cruise control disabling itself

• Cruise-control warning appearing on the instrument cluster

• Check Engine Light or service warning

Normal accelerator operation is often unaffected.

The vehicle can therefore drive normally even though the cruise-control system has been disabled.

Why Cruise Control May Be Completely Disabled

Cruise control is a safety-related system.

If the control module cannot reliably determine what the driver is requesting, it may disable the entire cruise-control function.

For example, imagine the system cannot distinguish between:

SET

and

RESUME

because both signals are outside their expected voltage ranges.

Allowing cruise control to continue operating under those conditions would not be desirable.

The safer strategy is usually to disable cruise control until the electrical problem is corrected.

Could the Cruise-Control Button Be Bad?

Yes, but it should not be the first assumption.

A damaged button can alter the resistance or electrical state of the input circuit.

For example, a switch that internally remains open may cause the control module to see the wrong voltage.

However, the same symptom can be caused by:

• Broken wiring

• Connector problems

• Clock-spring problems

• Incorrect switch assembly

• Steering-wheel module problems

Testing the signal is therefore more reliable than simply replacing the switch.

Could the Clock Spring Cause P0593?

Yes, on vehicles where the cruise-control switches are mounted on the steering wheel.

The steering wheel has to rotate while maintaining electrical connections to the buttons and other steering-wheel components.

This is accomplished through a clock spring or spiral cable assembly.

If one of its internal circuits becomes damaged, the cruise-control signal may no longer reach the control module correctly.

This can produce an abnormal input voltage and potentially trigger P0593.

If P0593 appeared together with other steering-wheel-related problems, the clock spring becomes particularly important to investigate.

For example, simultaneous problems with:

• Steering-wheel buttons

• Horn

• Cruise control

• Steering-wheel controls

can provide a strong clue toward the clock spring or its wiring.

Could a Short to Battery Cause P0593?

Yes.

If the Input B signal wire contacts a battery-voltage source, the control module can see a voltage significantly higher than expected.

This is a classic cause of a circuit high fault.

The technician should therefore check whether the signal wire has an unintended connection to a power supply.

But again, this is only one possibility. An open circuit can also produce a high reading depending on the design.

Could a Fuse Cause P0593?

A blown fuse is not usually the primary explanation for a circuit-high fault.

A fuse failure normally removes power from a circuit rather than making a signal excessively high.

However, the cruise-control system may share power supplies with other steering-wheel or control circuits, so the relevant fuses should still be checked.

The diagnostic result should determine what is actually happening rather than assuming that the fuse is responsible.

Could Low Battery Voltage Cause P0593?

Low system voltage is not the most obvious cause of a high input code.

Nevertheless, unstable electrical power can cause control modules to behave abnormally, particularly when several communication or voltage-related codes are stored at the same time.

If P0593 appears alongside multiple low-voltage, communication or module-reset codes, check the vehicle's battery, charging system, grounds and power supplies before replacing cruise-control components.

Can P0593 Be Caused by Wiring?

Absolutely.

Wiring should be high on the diagnostic list.

Look for:

• Broken conductors

• Damaged insulation

• Shorted signal wires

• Pinched wiring

• Pulled terminals

• Corrosion

• Loose connectors

• Previous wiring repairs

• Incorrect aftermarket connections

The wiring around the steering column deserves particular attention on vehicles where the cruise-control controls are located on the steering wheel.

What Happens Inside the Control Module?

The control module continuously monitors the cruise-control input.

It has an expected electrical range for each possible switch state.

If the measured voltage is outside the acceptable range for a certain period, the module can store P0593.

The logic is essentially:

Expected Input Range → Actual Input Voltage

If:

Actual voltage > permitted range

the module identifies the condition as Circuit High.

The exact thresholds and timing are manufacturer-specific.

Diagnosing P0593 Correctly

The first step is to determine what type of cruise-control input circuit the vehicle uses.

Do not begin by replacing the cruise-control switch.

Start with a complete scan.

Check whether other codes are stored, especially codes involving:

• Steering-wheel controls

• Clock spring

• Body control module

• Communication

• Reference voltage

• Battery voltage

Then check whether the cruise-control buttons produce any response in the scan tool's live data.

If the scanner shows the cruise-control switch states, this can be extremely useful.

For example, if every button appears as OFF, the problem may be different from a situation where the module permanently sees one particular switch command.

Check the Switch Signal

If the system uses a resistance-based switch network, measure the switch resistance according to the manufacturer's specifications.

Do not rely on generic resistance values found for another vehicle.

The correct values are specific to the switch and control module.

A button that should change resistance but produces no change can point toward a faulty switch assembly.

Check the Signal Voltage

The next step is to measure the voltage on the Input B circuit.

If the voltage is already high with no button pressed, investigate the circuit itself.

Then operate the relevant cruise-control buttons while monitoring the signal.

A healthy resistor-based system should normally produce predictable changes in voltage.

If pressing different buttons produces no change, the problem may be upstream or downstream of the switch.

Inspect the Clock Spring

If the controls are mounted on the steering wheel, inspect the clock spring when appropriate.

Do not condemn it simply because P0593 is present.

Instead, look for supporting evidence.

If several steering-wheel functions have stopped working simultaneously, the clock spring becomes much more suspicious.

Check the Wiring for a Short to Power

With the appropriate connectors disconnected and following the manufacturer's test procedure, check whether the Input B signal wire is unintentionally connected to a power supply.

If the signal wire has battery voltage when it should not, there may be:

• Chafed wiring

• Incorrect repair

• Pinched harness

• Internal connector damage

• Incorrect aftermarket wiring

Check for an Open Circuit

If the voltage is high because the circuit is open, continuity testing can help locate the interruption.

The technician should test the complete path:

Switch → Steering-Wheel Wiring → Clock Spring → Vehicle Harness → Control Module

A break anywhere along this path can prevent the expected signal from reaching the module.

Be Careful With Airbag-Related Components

Because steering-wheel wiring often passes through the clock spring and near the driver's airbag, diagnosis must follow the manufacturer's safety procedure.

Do not probe airbag circuits simply because you are investigating a cruise-control fault.

The cruise-control circuit should be identified from the wiring diagram before testing.

Can an Aftermarket Steering Wheel Cause P0593?

Yes.

An aftermarket steering wheel, replacement switch assembly, retrofit cruise-control buttons or incorrectly configured steering-wheel module can produce unexpected electrical values.

The control module expects a particular electrical strategy.

If the installed components use different resistance values or wiring, the module may interpret the signal incorrectly.

This is particularly relevant after steering-wheel modifications or module replacements.

P0593 After Clock Spring Replacement

If P0593 appears immediately after replacing the clock spring, inspect the installation before assuming that the replacement part is defective.

Possible issues include:

• Incorrect clock-spring part

• Connector not fully seated

• Pin damage

• Incorrect steering-angle calibration

• Incorrect harness routing

• Misalignment

• Damaged replacement component

The timing of the fault is important diagnostic evidence.

Is P0593 a Serious Fault?

P0593 is generally not a serious engine fault.

In most cases, the vehicle can continue to operate normally using the accelerator pedal.

The main consequence is loss of cruise-control functionality.

However, if P0593 is accompanied by steering-wheel, airbag, communication or other electrical faults, those additional problems should not be ignored.

Can You Drive With P0593?

Usually, yes, provided the vehicle drives normally and there are no other safety-related warnings.

The practical limitation is that cruise control may be unavailable or unreliable.

If the vehicle has an airbag warning, steering problem, electronic throttle warning or multiple electrical faults at the same time, it should be inspected promptly.

Does P0593 Mean the Cruise-Control Module Is Bad?

No.

The control module is only reporting that the signal it receives is outside the expected range.

The actual problem could be much farther upstream.

A logical diagnostic sequence is:

Switch → Wiring → Connector → Clock Spring → Steering-Wheel Module → Signal Circuit → Control Module

The module should generally be considered only after the external circuit has been proven correct.

P0593 vs. P0594

These codes point toward different parts of the cruise-control electrical system.

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

The control module is receiving a higher-than-expected signal from the multifunction input circuit.

P0594 – Cruise Control Servo Control Circuit/Open

The control module detects an open condition in the servo control circuit.

P0593 is therefore primarily concerned with the input command side, while P0594 concerns the servo/actuator control side.

This distinction is useful because replacing a cruise-control servo for P0593 would make little sense if the actual problem is a steering-wheel switch or signal wire.

Final Diagnostic Perspective

P0593 – Cruise Control Multi-Function Input "B" Circuit High is fundamentally an electrical signal problem, not a diagnosis of a specific failed component.

The control module is seeing an input voltage that does not match the expected range. That can result from a short to power, an open circuit, a damaged switch, a bad connector, a clock-spring problem, incorrect resistance or a control-module issue.

The most effective diagnosis is therefore to follow the signal from the driver's control to the module and determine where the expected voltage changes.

A good diagnostic sequence is:

Scan all modules → Check live cruise-control data → Identify Input B circuit → Inspect connectors → Test switch resistance/signal → Check clock spring → Test wiring for open/short → Verify reference and ground → Evaluate control module → Repair → Clear code → Test cruise control → Rescan

That approach avoids the common mistake of replacing the cruise-control switch or control module simply because P0593 is present.