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Technical Description of OBD-II Trouble Code
The range/performance of the Cruise Control Multi-Function Input B Circuit
What does that signify?
This is a generic diagnostic trouble code (DTC) for powertrain issues and is commonly found in OBD-II vehicles. This can include vehicles from various manufacturers such as Mazda, Alfa Romeo, Ford, Land Rover, Jeep, Dodge, Chrysler, Chevy, Nissan, etc. While it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
Cruise control was designed to enhance our comfort while driving. Its main purpose is to maintain a manually set vehicle speed (via steering wheel buttons, multifunction switch mounted on the steering column, etc.)
With cruise control, the driver no longer needs to control the gas pedal, and if the vehicle has an automatic transmission (A/T), it may also adjust shift points based on real-time driving conditions.
In general, this code suggests that the issue is related to the multifunction switch on the steering column. However, the location of the switch can vary significantly between manufacturers, so it is important to research accordingly. Checking the cruise control button is a good starting point to narrow down the possible locations. This code refers to a specific input circuit within the cruise control system. To determine which wire/circuit to focus on, consult the service manual and use the letters in the codes as a reference.
When there is a fault in the cruise control multi-function “B” input circuit, the engine control module (ECM) will illuminate the instrument cluster with the P0591 code and related codes (P0589, P0590, P0591, P0592, and P0593).
The letter “B” in this context could be used to distinguish a connector, wire, circuit group, etc. The manufacturer’s specifications are the best resource for this information.
The P0591 trouble code is set by the ECM when the values of the “B” cruise control multi-function input circuit are out of range or performing abnormally.
A cruise control stalk:
What is the severity of this DTC?
This DTC is generally considered to have low severity for most cruise control faults, although there can be exceptions. In most cases, this fault should not be a cause for serious concern. However, electrical issues tend to worsen over time. The cost of fixing this fault is usually moderate.
However, severity can be subjective, so it is recommended to compare prices and obtain multiple quotes and diagnosing strategies. Work orders alone may help in identifying the problem. It is always important to stay on top of general vehicle maintenance.
What are some of the symptoms of the code?
Symptoms of a P0591 trouble code may include:
What are some of the common causes of the code?
Some possible causes of the P0591 code include:
What are some P0591 troubleshooting steps?
Basic Step #1
Once you locate your vehicle’s multi-function/cruise control switch, visually inspect it for any contamination. Plastic buttons/switches can easily become dirty, greasy, or sticky, causing malfunctions or abnormal operation. Ensure that the switch operates smoothly. Address any problems immediately. If your scan tool/OBD reader allows it, observe the electronic operation of the switch through DATA STREAM (the mode name may vary between readers, but the important part is to find a mode that shows real-time data).
TIP: Avoid spraying cleaning solutions directly onto the button. Instead, lightly dampen a clean rag with water, soap & water, dash cleaner, etc., and carefully remove debris from the switch’s crevices. An air-powered blow gun can help remove debris without worsening the situation.
Basic Step #2
To access the connectors and harnesses involved in the cruise control/multi-function switch input circuit, you may need to remove some dash plastic/shrouds. Be careful when working with plastic. Comfortable room temperatures can make dash/interior plastic disassembly/assembly easier.
If you can easily access the connector, you may be able to follow specific troubleshooting steps provided by your service manual. Switch testing will likely involve recording electrical values using a multimeter. This may include operating the switch while recording and/or static testing. Consult the specific make and model’s service manual for a “diagnostic flow chart”.
Basic Step #3
A problem with the ECM is usually the last resort in diagnosis. Given that it can be one of the most expensive electronic repairs for a vehicle, it is best to leave this to the professionals.
Professional Service
While we always recommend resolving the issue mechanically, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
