What does it mean?
Many modern vehicles come with cruise control systems (CCS), which sets and maintains vehicle speed. Some CCS can be set to maintain certain distance behind the lead vehicle, and can even autonomously brake in case the lead vehicle suddenly slows. On long commutes, CCS can improve traffic flow and fuel economy, as well as make it easier on the driver.
The CCS may or may not be integrated with the engine control module (ECM), usually depending on whether or not the vehicle is equipped with an electronic throttle control system (ETCS) or “fly-by-wire” throttle. Regardless of type, the CCS monitors driver input, usually via a multi-function stalk or buttons on the steering wheel, and vehicle speed, modulating throttle position to maintain vehicle speed.
Advanced systems may also monitor, via sonar or laser ranging systems, vehicles in front, to maintain following distance, and may also modulate electro-hydraulic braking. For the purposes of this discussion, we’re focusing on the driver controls. If the ECM detects a problem with the CCS multi-function switch, such as receiving two inputs at the same time or being unable to detect its neutral position, it will disable cruise control and set a diagnostic trouble code (DTC) in system memory.
DTC P0591 is defined as “Cruise Control Multi-Function Input “B” Circuit Range/Performance.”