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P0591 OBD-II Trouble Code: Cruise Control Multi-Function Input "B" Circuit Range/Performance

check engine light

P0591 code definition

When the P0591 code gets stored, it’s because the PCM has learned of an electrical malfunction stemming from the cruise control multi-function input circuit.

What the P0591 code means

On most vehicles, the PCM works with the cruise control module to automatically control vehicle speed. If the PCM receives input that the vehicle’s speed can no longer be controlled, it will trigger a self test of the cruise control system. Code P0591 is recorded when it’s found that an irregular voltage or level of resistance occurred in the cruise control multi-function input circuit.

What causes the P0591 code?

The majority of the time, the reason this code gets stored is because there’s a faulty cruise control switch that has failed to work.

Another common reason is circuitry located in the cruise control buttons that has shorted or is open because of spilled liquids. It takes surprisingly little to bring cruise control operations to a complete halt.

Other reasons for code P0591 include:

  • Corroded or otherwise damaged connectors
  • Open or shorted wiring
  • Blown fuses

What are the symptoms of the P0591 code?

The most obvious symptom associated with P0591 is that the cruise control simply doesn't work anymore. In most cars, a Check Engine Light may be your first clue that something is wrong, though. In other vehicles, it takes about three incidents before that light comes on. Some makes and models don’t make use of the Check Engine Light for this reason. As we mentioned in the last section, blown fuses may accompany the problems that cause this code.

How does a mechanic diagnose the P0591 code?

Your mechanic will begin their diagnosis by using an OBD-II scanner to confirm which codes have been stored by your vehicle’s PCM. Once they know the codes they’re dealing with, they’ll probably take a look at the state of the wiring and other electrical components related to the cruise control. This would include the fuses.

After that, if the cause hasn’t been found, they will examine the vacuum supply hoses responsible for supplying the cruise control module. If still no signs of the problem are found, they will backtrack from the hoses and continue their inspection until they find the issue.

Common mistakes when diagnosing the P0591 code

If the mechanic doesn’t take the time to look for blown fuses, they will almost certainly make a mistake with their diagnosis. That’s because they’ll blame components for being faulty when really it’s just a matter of energy supply. After pricey replacements, your cruise control still won’t work and – because the blown fuse was never addressed – other components still won’t either.

How serious is the P0591 code?

It’s probably a good idea to get P0591 looked at right away so you can get back to using your cruise control. That being said, it’s not the type of problem that puts you and your passengers in any danger.

Still, if it turns out that it has something to do with the vacuum supply hoses or some other component that operates other parts of the vehicle too, you’ll be glad you saw a mechanic immediately. This minor problem could spread and turn into something worse.

What repairs can fix the P0591 code?

Your mechanic will do some combination of these measures to fix the P0591 code:

Additional comments for consideration regarding the P0591 code

If the problem turns out to be a blown fuse, it needs to be fixed, but don’t stop there. The fuse may have caused other problems that didn’t have codes stored for them yet.

Need help with a P0591 code?

For the help of a certified mobile mechanic, YourMechanic is here to help. Just fill out this online form for a free quote and to book your appointment. Otherwise, you can also call 1-800-701-6230 to speak with a service advisor. We’ll dispatch your mechanic ASAP to either your home or business where they’ll fix your vehicle onsite.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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