Description of the P0591 fault code.
OBD2 Error Codes

P0591 Cruise control multifunction switch circuit "B" input range/performance

26.05.2022 /

P0591 – OBD-II Trouble Code Technical Description

Trouble code P0591 indicates that the PCM has detected an electrical fault in the cruise control multifunction switch input circuit “B”.

What does the fault code mean P0591?

Trouble code P0591 indicates an electrical problem in the cruise control multifunction switch input circuit “B”. This means that the engine control module (PCM) has automatically detected unusual voltage or resistance in this circuit, which may cause the cruise control system to not operate properly. If the PCM detects that the vehicle can no longer automatically control its own speed, a self-test will be performed on the entire cruise control system. The P0591 code will appear if the PCM detects that the voltage and/or resistance in the cruise control multifunction switch input circuit is abnormal.

Possible reasons

Several possible reasons for the P0591 trouble code:

  • Damaged or broken wiring: The wiring connecting the cruise control multifunction switch to the PCM may be damaged, broken, or corroded, causing abnormal voltage or resistance in the circuit.
  • Multifunction switch malfunction: The switch itself or its internal contacts may be damaged, causing incorrect signals to be sent to the PCM.
  • Malfunctioning PCM: The engine control module may be damaged or have software errors, causing signals from the multi-function switch to be incorrectly detected.
  • Grounding problems: Insufficient grounding of the cruise control system or PCM can also cause unstable voltage or resistance in the circuit.
  • Electrical interference: There may be external electrical noise or interference that could affect the operation of the cruise control system and cause DTC P0591 to appear.
  • Malfunction of other components of the cruise control system: Problems with other components such as speed sensors or actuators can also cause this error to appear.

To accurately determine the cause, it is recommended to carry out diagnostics using a diagnostic scanner and check the relevant components according to the repair manual for the specific make and model of the vehicle.

What are the symptoms of a fault code? P0591?

Symptoms when trouble code P0591 is present may include the following:

  • Cruise control malfunction: One of the most obvious symptoms is the cruise control system not working or not working properly. This may manifest itself as an inability to engage cruise control, an inability to set or change the cruise control speed, or other anomalies in its operation.
  • Appearance of Check Engine Light (CEL): It is possible that the Check Engine Light will be activated. This may be the result of the PCM self-diagnosis detecting a malfunction in the cruise control system.
  • Loss of power or poor fuel economy: In some cases, a malfunction in the cruise control system may result in loss of engine power or increased fuel consumption due to improper operation of the engine management system.
  • Unstable or abnormal vehicle behavior at speed: This may include unpredictable changes in speed or traction, which may be due to the cruise control system not operating properly.
  • Other fault codes: It is possible that in addition to P0591, other trouble codes related to the operation of the cruise control system or PCM may also appear.

If you notice any of these symptoms or if the Check Engine Light is activated, it is recommended that you take it to a qualified auto mechanic for diagnosis and repair.

How to diagnose a fault code P0591?

To diagnose DTC P0591, follow these steps:

  1. Checking Error Codes: Use a diagnostic scan tool to read error codes from the PCM memory. If a P0591 code is detected, this will be a key indicator to begin diagnosing.
  2. Checking the cruise control: Check the operation of the cruise control system. Make sure cruise control can be turned on, set and hold speed can be changed. Any unusual anomalies should be noted.
  3. Checking electrical connections and wiring: Inspect the wiring and connectors connecting the cruise control multifunction switch to the PCM. Make sure the wiring is not damaged, broken or showing signs of corrosion. Also check the pins in the connectors for bad connections.
  4. Checking the status of the multi-function switch: Check the status of the cruise control multi-function switch. Make sure the switch is working properly and has no visible damage.
  5. Using a multimeter: Use a multimeter to check the voltage and resistance in the “B” input circuit of the multi-function switch. Compare the values ​​obtained with the vehicle manufacturer's recommendations.
  6. Check PCM: If all the above steps do not resolve the problem, then the problem may be in the PCM. However, PCM testing requires specialized equipment and procedures, so it's best to hire a professional.

After completing these steps, you can determine the cause and resolve the problem causing the P0591 code.

Diagnostic errors

When diagnosing DTC P0591, the following errors may occur:

  • Incorrect interpretation of code: A mechanic may misinterpret the meaning of the P0591 code and focus on the wrong components or systems.
  • Insufficient electrical circuit check: Poor checking of wiring and connectors may occur, which may result in missing the root cause of the problem.
  • Skipping important diagnostic steps: Necessary diagnostic steps such as contact testing, voltage and resistance measurements, etc. may be missed, which may result in the cause of the fault being incorrectly determined.
  • Ignoring other potential problems: A mechanic may only focus on the problem with the multifunction cruise control switch without paying attention to other potential causes of the P0591 code, such as wiring or PCM problems.
  • Malfunction of diagnostic equipment: Using faulty or outdated diagnostic equipment may result in incorrect results or an inability to accurately determine the cause of the error.
  • Inexperience or lack of qualifications of the mechanic: Incorrect diagnosis due to inexperience or lack of qualifications of the mechanic can also lead to errors.

To successfully diagnose and resolve the P0591 error, it is important to have professional skills, the correct equipment, and follow the manufacturer's recommendations for diagnostic procedures. If you do not have experience diagnosing automotive systems, it is recommended that you contact a qualified auto mechanic or service center.

How serious is the fault code? P0591?

The severity of the P0591 trouble code may vary depending on the specific situation and operating conditions of the vehicle. Here are several factors that may influence the severity of this error:

  • Effect on cruise control operation: If the cruise control system is not working due to a P0591 code, it can be quite annoying, but is usually not a critical driving safety issue.
  • Potential Fuel Economy Problems: Incorrect operation of cruise control or other PCM-controlled systems may adversely affect fuel economy and overall vehicle performance.
  • Losing control of speed: In some cases, the P0591 code can cause you to lose control of your speed, which can create a dangerous driving situation, especially on highways.
  • Impact on other vehicle systems: Incorrect operation of the PCM or multi-function switch may also affect other vehicle systems, which may result in poor overall performance or safety.

Overall, although P0591 is not usually an emergency or critical problem, it should be taken seriously and diagnosed and repaired as soon as possible to avoid further problems with your vehicle's control system.

What repair will help eliminate the code? P0591?

Troubleshooting trouble code P0591 may include the following steps:

  1. Checking and replacing the multi-function cruise control switch: If diagnostics have been carried out and the cause of the error is found to be related to the multi-function switch, it should be checked for damage and, if necessary, replaced with a new one.
  2. Checking and repairing electrical connections and wiring: Check the electrical connections and wiring between the multifunction switch and the PCM. If damage, broken wires or corrosion are found, they should be repaired or replaced.
  3. Check and replace the PCM: In rare cases, the problem may be due to a faulty PCM. If all other components are checked and in good condition and the problem still persists, the PCM may need to be replaced or reprogrammed.
  4. Additional diagnostic procedures: If necessary, additional diagnostics may be required to identify other problems related to the operation of the cruise control system or PCM.
  5. Software testing and updating: In some cases, the problem may be related to the PCM software. In this case, the PCM software may need to be tested and updated.
  6. Follow-up diagnostics and testing: After repair work, it is recommended to re-read the fault codes and conduct a test drive to ensure that the problem has been resolved.

It is important to have diagnostics and repairs performed by a qualified auto mechanic to avoid additional problems and ensure the cruise control system and PCM are functioning properly.

What is P0591 Engine Code [Quick Guide]

P0591 – Brand-specific information

Trouble code P0591 can occur on different makes of vehicles. Here is a list of some brands with their decodings:

These are just a few examples of car brands where trouble code P0591 may occur. The exact meaning and cause of the error may vary depending on the model and year of the vehicle. If this error occurs, it is recommended that you consult the repair manual for your specific vehicle brand for more detailed information and recommendations for troubleshooting.

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