Cruise Control Multi-Function Input B Circuit Range/Performance
Your cruise control's secondary input button or switch is sending a signal that's outside its normal range, like a radio stuck between stations. The ECU can't properly read cruise control commands because the circuit voltage is too high or too low.
The ECM monitors voltage on the cruise control multi-function input B circuit (typically a potentiometer or switch array on the steering wheel). It expects voltage to stay within a defined range (usually 0.5V–4.5V) and transition smoothly when buttons are pressed. Out-of-range voltage indicates a wiring fault, switch failure, or loose connector.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Input B Voltage | 0.5V to 4.5V with smooth transitions | Below 0.2V or above 4.8V; erratic fluctuations |
| Signal Stability | Steady state within 100ms response time | Voltage jumping or stuck at extreme values |
Code P0591 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0591 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.