What Does P256C Mean?
The PCM detects abnormally low voltage on the idle speed selector sensor/switch circuit. The signal is below the minimum threshold expected for any valid switch position.
Common Causes
40%
Short to ground in wiring harness
30%
Internally shorted idle speed selector switch
15%
Damaged or corroded connector causing ground short
10%
Chafed wiring rubbing against chassis ground
5%
PCM internal circuit failure pulling signal low
Diagnostic Steps
1
Step 1: Disconnect idle speed selector switch and observe if voltage returns to normal at PCM connector
2
Step 2: Measure voltage at switch connector with switch disconnected (should read reference voltage)
3
Step 3: Check for continuity between signal wire and ground with switch disconnected
4
Step 4: Inspect wiring harness routing for pinch points, sharp edges, or contact with hot components
5
Step 5: If no shorts found in harness, test switch internal resistance to ground
Estimated Repair Cost
$80 - $450
Parts + labor, varies by vehicle and location
The PCM detects abnormally low voltage on the idle speed selector sensor/switch circuit. The signal is below the minimum threshold expected for any valid switch position.
The most common cause of P256C (Engine Idle Speed Selector Sensor/Switch Circuit Low) is: Short to ground in wiring harness
Typical repair costs for P256C range from $80 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle remains safe to operate. Default idle speed will be used, which may be higher or lower than desired for specific conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P256C to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Idle Control
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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