P256A
Engine Idle Speed Selector Sensor/Switch Circuit/Open
The idle-speed selector input is open. On commercial vehicles this switch lets the operator pick a fast-idle setpoint (for example to keep auxiliary loads supplied). With the input broken, the ECM cannot apply an operator-selected idle speed.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Symptoms
- Check engine light on
- Idle speed too high, too low, or unstable
- Possible stalling at idle or after cold start
Common causes
Ordered by how often each turns out to be the cause.
- Open wire or unplugged switchCommon
- Pin pushed back out of connector bodyLess likely
- Cracked solder joint inside the module or sensorLess likely
Parts involved
- Idle Speed Selector Switch
- Wiring Harness
P256A is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.