A faulty idle speed selector switch or sensor internally shorted to ground.; Damaged or chafed wiring in the selector circuit, causing a short to ground.; Corroded or loose electrical connectors at the switch or PCM.; Water intrusion into the switch or connector, creating a low-resistance path.; A failed PCM driver or internal circuit issue (less common).; Aftermarket accessories improperly tapped into the idle selector circuit.

DTC codes / P25xx · Vehicle speed, idle control and auxiliary inputs / P256C

P256CEngine Idle Speed Selector Sensor/Switch Circuit Low

Q: What does code P256C mean?

Engine Idle Speed Selector Sensor/Switch Circuit Low. P256C indicates that the powertrain control module (PCM) has detected a low-voltage condition on the engine idle speed selector sensor or switch circuit. This selector is typically part of the idle control system, allowing the driver or system to choose an idle speed strategy, and the low signal means the PCM sees the circuit pulled to ground or below the expected threshold.

P256C indicates that the powertrain control module (PCM) has detected a low-voltage condition on the engine idle speed selector sensor or switch circuit. This selector is typically part of the idle control system, allowing the driver or system to choose an idle speed strategy, and the low signal means the PCM sees the circuit pulled to ground or below the expected threshold.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

The check engine light illuminates, often with no other noticeable symptoms.
Idle speed may be stuck at a default or high value, or may fluctuate unexpectedly.
The idle speed selector switch may feel unresponsive or fail to change idle characteristics.
In some vehicles, the engine may stall when coming to a stop or idling roughly.
Remote start or idle-up features may not function correctly.
A message may appear on the driver information center indicating an idle control fault.

Common causes, most likely first

A faulty idle speed selector switch or sensor internally shorted to ground.
Damaged or chafed wiring in the selector circuit, causing a short to ground.
Corroded or loose electrical connectors at the switch or PCM.
Water intrusion into the switch or connector, creating a low-resistance path.
A failed PCM driver or internal circuit issue (less common).
Aftermarket accessories improperly tapped into the idle selector circuit.

How to diagnose and fix it

Verify the code with a scan tool and check for any related codes in other modules.
Inspect the idle speed selector switch and its connector for corrosion, damage, or loose pins.
Disconnect the switch and measure resistance across its terminals; compare to specifications.
Check the wiring harness from the switch to the PCM for shorts to ground using a multimeter.
Test the switch signal at the PCM connector to confirm whether the fault is in the wiring or the PCM.
Replace the faulty switch or repair the damaged wiring as needed.
Clear the code and perform a relearn or idle adaptation procedure if required by the manufacturer.

Idle Selector Circuit Low: Hidden Causes

A common oversight is assuming the switch itself is bad when the real problem is a chafed wire shorting to ground behind the dashboard or near the pedal assembly.
The idle speed selector is often integrated with the accelerator pedal position sensor on some drive-by-wire systems, so a low circuit code can be triggered by pedal sensor issues rather than a separate switch.
Aftermarket remote start or alarm systems that tap into the idle circuit can introduce a low-resistance path and set this code intermittently.
On vehicles with a manual transmission, the clutch pedal position switch shares a ground with the idle selector on some models, and a faulty clutch switch can cause a false low signal.
This code does not always mean the switch is defective; a low battery or charging system fault can cause the PCM to misinterpret the selector signal during low-voltage conditions.

Can I keep driving with P256C?

The vehicle is generally safe to drive with this code, but idle quality may be affected, so avoid prolonged idling in traffic and be prepared for possible stalling. Have the fault diagnosed soon to prevent drivability issues.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.