OBD-II code P0591 indicates the powertrain control module (PCM) has detected an out-of-range voltage or performance issue in the cruise control multi-function input 'B' circuit. This typically involves the signal from the cruise control switch or related wiring.
The multi-function switch (often integrated into the turn signal or steering column stalk) is the most common failure point. Wear, corrosion, or internal damage can cause out-of-range signals.
Wiring harnesses between the cruise control switch and PCM, especially near the steering column or under the dash, are prone to chafing, corrosion, or loose connections.
Insufficient voltage or poor grounding to the cruise control switch or related circuits can cause signal performance issues.
In vehicles with steering wheel-mounted cruise control buttons, the clockspring (ribbon cable) can wear out or break, disrupting the signal.
While uncommon, a PCM software glitch or internal fault in the cruise control input circuit could trigger this code. This is a last-resort diagnosis after all other causes are ruled out.
No. The PCM is rarely the cause of P0591. In most cases, the issue lies with the cruise control switch, wiring, or connectors. PCM replacement should only be considered after all other potential causes have been thoroughly tested and ruled out.
Yes, the vehicle is typically drivable, but the cruise control system will likely be inoperative. Address the issue promptly to restore cruise control functionality and prevent potential electrical issues from worsening.
Intermittent operation often points to a failing cruise control switch, loose wiring, or a damaged clockspring. These components may make sporadic contact, causing the system to work briefly before failing again.
While a scan tool is helpful for retrieving codes and monitoring live data, a multimeter and wiring diagram are often sufficient for diagnosing the root cause of P0591. Advanced scan tools may provide additional insights but are not always required.