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Error Code P0591 is defined as Cruise Control Multi-Function Input B Circuit Range/Performance. This code is a generic trouble code that typically applies to vehicles equipped with the OBD-II system. It’s particularly common among Alfa Romeo, Chevrolet, Chrysler, Ford, Dodge, Jeep, Land Rover, Mazda, and Nissan. Specifications on the definition, troubleshooting, and steps for repairs, of course, vary depending on the make, year, model, and powertrain configuration of the vehicle.
Cruise control is a feature that provides comfort and convenience to the vehicle riders. Its purpose is to keep the vehicle at a specific speed, which is set by the driver manually (through steering wheel buttons, multifunction (i.e., signal lights, cruise, wipers, etc.) switch mounted on the steering wheel column, etc.)
If the vehicle is equipped with the A/T (automatic transmission) however, the driver no longer has to control the gas pedal. It can also control and adjust shift points depending on the demands of the driver’s real-time driving condition.
Generally, this code refers to the multifunction switch of the steering column. With that said, location varies from one make and model to another. Thus it’s important to check thoroughly. The cruise control button is the best place to start and narrow your search. This code usually refers to a specific input circuit within the cruise control system. Thus, make sure you check with the service manual to determine which wire/circuit to focus on and test.
The “B” in the description is used to distinguish a connector, circuit group, wire, etc. With that said, it’s still best to check with the manufacturer’s specifications.
Error Code P0591 specifically means the PCM (powertrain control module, also known as ECM in other vehicle makes) reads that the “B” cruise control multi-function input circuit values are either out of range or performing abnormally.
Other related error codes include:
As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. Other common symptoms include:
- Inoperative cruise control
- Cruise control light activated regardless of switch position
- Cruise control can’t be set to desired speed
- Abnormal vehicle speed when cruise control is activated
In most cases, this code is caused by a defective or damaged cruise control switch, specifically is there is liquid spilled on the cruise control button, or there’s a dust debris buildup on the button. Other possible causes include:
- Stuck, broken, or missing cruise control button
- Wiring problems such as chafing wires on the steering column or dash parts, corrosion, water intrusion, etc.
- Malfunction on the connector (broken plastic tabs, corroded pins, swollen connector body, etc.)
- Dirt, fluid, grime in cruise control button, causing mechanical malfunction
- A PCM problem (water intrusion in the computer’s body, internal overheating, short, etc.)
How to Check
Check for any contamination as soon as you have located your vehicle’s multi-function/cruise control switch. It is quite easy for plastic buttons to get dirty, greasy, and sticking, causing it to malfunction and work abnormally. Make sure the switch’s mechanical operation is smooth and seamless. Any problem must be addressed as soon as possible. If your scan tool/OBD reader has the capabilities, feel free to check via the DATA STREAM (mode name varies between readers) to check the real-time data.
Tip: Never spray cleaning solutions directly to the button. Rather, dampen a cleaning rag lightly using water, or mixture of soap and water, a dash cleaner, etc., and then carefully remove dirt debris through the crevices of the switch. You can also use an air-powered blow gun to remove dirt.
Get access to the connector and harness within the cruise control/multi-function switch input circuit. For some vehicles, you may have to remove some shrouds or dash plastic. Be extra careful when working with this plastic. Work at comfortable room temperature with disassembling or assembling the plastic dash/interior.
If you can access the connector now, you can check with the specific troubleshooting steps from your service manual. Switch test is a common scenario to record electrical valve using your multimeter. This may include operating the switch while recording and static testing. Again, refer to the specific make and model of the service manual for the proper “diagnostic flow chart.”
PCM problem is usually the last resort for diagnosis of this code, and it is also the most expensive. Thus, you’re better off to leave it with the professionals.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Replacement of the defective cruise control switch
- Repair or replacement of damaged electrical components
- Cleaning of connectors, tightening connections
- Replacement of burnt fuses
This code doesn’t pose a serious problem or safety hazards, as it can only affect the cruise control feature of the vehicle. However, if part of the problem is a blown fuse, then it may be a sign of a more serious issue that may lead to safety hazards when driving, such as rending turn signal lights inoperable. If this is the case, then the code must be addressed as soon as possible.