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 circuit of the idle speed selector sensor or switch. This sensor is typically part of the throttle body or idle air control system and helps the ECM determine the desired idle speed based on driver input (e.g., idle up switch for high idle). A low circuit voltage can be caused by a faulty sensor, wiring issues, or a short to ground, leading to incorrect idle speed control.
1. 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)
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Erratic idle speed or stalling
- Inability to adjust idle speed (e.g., high idle not working)
- Reduced engine performance
3. 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 a short to ground by disconnecting the sensor and measuring continuity between the signal wire and ground.
Step 5: If wiring and sensor are okay, test the ECM output voltage at the sensor connector; if missing, suspect ECM issue.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive with this code as it can cause unstable idle, stalling, or poor drivability, which may lead to unsafe driving conditions.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for diagnosis and repair, depending on whether the sensor needs replacement or wiring repair.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.