P0591 — Cruise Control Multi-Function Input « B » Circuit Range/Performance

The diagnostic trouble code P0591 indicates a problem with the cruise control system, specifically related to the « Multi-Function Input ‘B' » circuit. This code suggests that the vehicle’s Engine Control Unit (ECU) has detected an issue with the signal or the circuit that communicates with the cruise control system. Note that label variations may occur between different vehicle manufacturers.

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Manufacturer variations for trouble code P0591

Vehicle manufacturers often use different labels or codes to identify similar issues related to the cruise control system. The variations for P0591 include:

• Ford: U2000 — Cruise Control Switch Circuit Malfunction
• GM (Chevrolet, GMC, Cadillac): C0570 — Cruise Control Electrical Circuit
• Honda: B2732 — Cruise Control Main Switch Circuit Malfunction
• BMW: 5F55 — Cruise Control Signal Error
• Volkswagen / Audi: 00459 — Cruise Control Switch Input Signal Missing / Faulty

It’s important to consult the specific manufacturer’s wiring diagrams and service documentation when diagnosing this code, as labels and error descriptions can vary.

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What does trouble code P0591 mean?

In simple terms, P0591 indicates that the vehicle’s ECU has detected an inconsistency or fault in the « Multi-Function Input ‘B' » circuit related to the cruise control system. This circuit is responsible for transmitting signals from the cruise control switches and controls to the ECU. When the signals are not within expected ranges, or if there’s an open, short, or poor connection, the ECU registers this as a performance issue and triggers the code.

The code most often appears when there is a wiring problem, a faulty switch, or an issue within the cruise control module itself. It may happen during or after active cruise control operation, or sometimes even when the cruise control is not engaged but the system detects a fault in the circuit.

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Severity and risks of trouble code P0591

P0591 is considered a moderate diagnostic trouble code. While it primarily affects cruise control functionality, it does not typically interfere directly with vehicle safety or fundamental driveability. However, neglecting this issue could lead to further electrical faults or malfunctions, potentially impacting related systems or causing the cruise control system to deactivate unexpectedly.

In most cases, the vehicle can still be driven, but cruise control will be unavailable until the fault is resolved. Persistent driving with unresolved electrical issues may increase the risk of additional damage or unrelated performance problems, especially if associated wiring or components deteriorate further.

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Symptoms of trouble code P0591

The symptoms associated with P0591 can vary depending on the overall condition of the vehicle and its systems, but common indicators include:

• Cruise control not engaging or disengaging unexpectedly
• Warning light on the dashboard, such as the cruise control or check engine light
• Inconsistent cruise control operation
• Failure of cruise control system to stay active during highway driving
• Possible electrical noises or irregular signals from the steering wheel controls or stalks

Drivers may also notice no immediate change in the vehicle’s ability to drive or accelerate, but the cruise control system’s inoperability can impact comfort and long-distance driving convenience.

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Most likely causes of trouble code P0591

This code typically results from issues within the cruise control circuitry or related components. The following causes are ranked from most to least common:

1. Faulty or damaged cruise control switch or multi-function switch — The switch that activates or controls cruise functions may have internal faults or wear that interrupt the signal transmission.
2. Wiring or connector problems in the « Multi-Function Input ‘B' » circuit — Corrosion, damaged wiring, or poor connections can lead to inconsistent signals or open circuits.
3. Malfunction or failure of the cruise control module or electronic control module (ECU) — The control module itself might be experiencing internal faults or software glitches.
4. Bluetooth or alternative control device conflicts — In some vehicles equipped with advanced controls, incompatible or faulty auxiliary devices can interfere with signals.
5. Other electrical issues or short circuits within the cruise control circuit — Broader electrical system problems may contribute to signal anomalies.

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How to diagnose trouble code P0591

Diagnosing P0591 involves a systematic approach to identifying circuit faults or component malfunctions. Here are straightforward diagnostic steps:

1. Visual inspection of wiring and connectors — Check all visible wiring for signs of wear, corrosion, or damage, focusing on the cruise control switch and associated harnesses.
2. Verify the condition of the cruise control switch or stalk — Test the switch physically for proper operation and inspect for any external damage.
3. Use live data to observe signals — Connect an OBD-II scan tool capable of real-time data; monitor the « Multi-Function Input ‘B' » circuit signal when the switch is activated.
4. Check circuit continuity and resistance — Use a multimeter to verify continuity across wiring and compare resistance values with manufacturer specifications for known good circuits.
5. Inspect the cruise control module or control unit — If feasible, examine the module for signs of water ingress, damage, or faulty solder joints. In some cases, a module reset or reprogramming may be necessary.
6. Clear DTCs and test drive — After repairs or component replacement, clear codes and perform a test drive to confirm proper operation and absence of the code.

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Possible repairs for trouble code P0591

Addressing P0591 generally involves targeted repairs based on the identified cause. Common solutions include:

• Replacing the cruise control switch or multi-function stalk — This is often a simple, basic repair performed by DIY enthusiasts or professionals.
• Repairing or replacing damaged wiring or connectors — Restoring wiring integrity, removing corrosion, or securing loose connections should be performed to ensure reliable signals.
• Replacing the cruise control control module — This may require professional diagnosis and programming, especially for newer or complex systems.
• Performing software updates or ECU reprogramming — Evidence of software faults causing the issue can be addressed via OEM updates, typically handled by authorised technicians.
• Electrical system diagnosis and repair — Broader electrical issues, such as fuse replacement or wiring harness replacement, may be necessary if fault isolation indicates systemic problems.

Most repairs related to this code are either straightforward or require professional expertise, especially when dealing with the control module or wiring harnesses.

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Vehicles commonly associated with trouble code P0591 in Europe

In Europe, P0591 has been reported across a variety of makes and models, especially those equipped with electronic cruise control systems. Some of the most affected brands and model families include:

While specific data can vary and is subject to change, typical vehicles include:

• Ford: Focus, Mondeo, Kuga
• Volkswagen / Audi: Golf, Passat, A3
• Peugeot / Citroën: 308, C4, C5
• BMW: 1 Series, 3 Series
• Mercedes-Benz: C-Class, E-Class

It’s essential to check each vehicle’s specific wiring and control system details during diagnosis, as the architecture may differ between models and years.

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Frequent mistakes with trouble code P0591

Diagnosing and repairing P0591 can sometimes lead to common pitfalls. Recognising these can improve diagnostic efficiency:

• Misinterpreting the code as a simple switch fault without inspecting wiring — Always verify wiring and connectors before replacing components.
• Replacing components without confirming circuit faults — Carry out continuity and resistance tests rather than assuming component failure.
• Ignoring related electrical problems — Electrical circuits are interconnected; other faults may mask or cause the P0591 code.
• Overlooking software or calibration updates — Sometimes the fault is due to software; ensure all vehicle firmware is up to date before replacing hardware.
• Failing to clear codes after repairs — Always clear the DTCs and perform a test drive to verify the repair was successful.

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FAQ — trouble code P0591

Can this code disappear on its own? –

It is unlikely for P0591 to disappear spontaneously. Most often, a physical issue such as wiring damage or a faulty switch needs to be addressed. However, sometimes a temporary disconnect or interference can cause the code to reset if the underlying cause is resolved.

Can I keep driving? –

Yes, in most cases, you can continue driving if the cruise control system is inoperative, but the vehicle’s core driveability should not be affected. It is advisable, nevertheless, to have the issue diagnosed and repaired promptly to restore full vehicle functionality and avoid further electrical problems.

Why does the code return after clearing it? –

If the underlying cause (such as wiring issues or a faulty switch) persists, the code will reappear after clearing. This indicates that the fault has not been repaired adequately. Proper diagnosis and component replacement are necessary for a lasting fix.