Engine Idle Speed Selector Sensor/Switch Circuit Low
The P256C code indicates that the Engine Control Module (ECM) has detected a low voltage condition in the idle speed selector sensor or switch circuit. This sensor/switch is typically used in diesel engines to allow the driver to select a higher idle speed for auxiliary equipment operation. A low voltage reading suggests a short to ground, open circuit, or faulty sensor/switch, which can prevent the ECM from properly adjusting idle speed.
01. Most Common Causes
- Faulty idle speed selector sensor/switch
- Short to ground in the sensor circuit
- Open or damaged wiring/connectors
- Corroded or loose electrical connections
- Failed ECM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Inability to adjust idle speed via selector
- Engine idle speed stuck at default or erratic
- Reduced engine performance or stalling
03. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm P256C and check for other codes.
Step 2: Visually inspect the idle speed selector sensor/switch and its wiring for damage, corrosion, or loose connections.
Step 3: Test the sensor/switch resistance and voltage with a multimeter; compare to manufacturer specifications.
Step 4: Check for shorts to ground or opens in the circuit by disconnecting the sensor and testing continuity.
Step 5: If all tests pass, suspect a faulty ECM; perform a reflash or replacement if necessary.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is generally safe to drive short distances, but the engine may not idle properly, which can affect drivability and fuel economy. Have it diagnosed promptly.
How much does it cost to fix?▼
Repair costs typically range from $50 to $150 for parts and labor, depending on the cause. Sensor replacement is usually under $100, while wiring repairs may be less.