Fault code P256A – engine idle speed selector sensor/switch circuit open

Fault code P256A is called “Engine Idle Speed Selector Sensor/Switch Circuit Open” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P256A

OBD-II fault code P256A appears when the PCM detects an open circuit in the sensor or idle speed switch. Set when the transmission control module is unable to maintain or control engine speed at idle.

Fault code P256A – engine idle speed selector sensor/switch circuit open

In most cases, the PCM will attempt to make adjustments to various systems such as fuel adjustment, ignition angle, injection pulse width, and others. Trying to maintain the idle speed set point. However, when this limit is reached, the PCM will set code P256A and turn on the warning lamp.

The idle speed in gasoline engines is regulated and controlled by the idle speed control valve, which is usually located on the throttle body. The valve is driven by a stepper motor, which receives commands from the PCM based on information received from various sensors located on the engine.

As long as the throttle remains closed, speed and idle quality are controlled by the PCM through the idle control valve.

For example, if the engine is started in sub-zero temperatures, the PCM commands the valve to allow less air to enter the engine. This enriches the fuel-air mixture and creates conditions for cold-weather idle control.

As the engine warms up and combustion becomes lighter and more complete. The PCM commands the idle control valve to increase the amount of air flowing into the engine according to changing conditions.

If the load on the engine changes, such as when some electrical consumers are turned on or off. The PCM adapts the signal voltage to the idle control valve. To ensure that the idle speed remains stable, and at the rpm level specified by the manufacturer.

Note that the idle air control valve is closed by the PCM when the throttle is opened. This is done to prevent excess air from entering the engine.

Symptoms of vehicle malfunctions

The main signal that an error P256A has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check Engine” control lamp on the control panel will light up (the code will be recorded in the memory as a fault).
  2. Floating revolutions, as well as attempts to stall at idling speed.
  3. Idle speed is higher or lower than normal.
  4. The engine may stall if the idle speed is not maintained by pressing the accelerator pedal.
  5. Unusual noises from the throttle body area.
  6. Other IAC related trouble codes.

Depending on the symptoms, the severity of the P256A code can range from moderate to severe. Therefore, it is recommended that the problem be corrected as soon as possible.

Factors that can cause this error code

The error code P256A can mean that one or more of the following problems have occurred:

  • The idle speed sensor is defective.
  • The idle speed switch is defective.
  • Sensor or regulator connector is damaged.
  • Damaged, burned, shorted, or corroded wiring.
  • Defective stepper motor.
  • Leaking vacuum in motor.
  • Carbon buildup in throttle body or IAC valve.
  • Contaminated mass air flow sensor.
  • Sometimes faulty PCM module is the cause.

How to fix or reset OBD-2 code P256A

Some suggested steps for troubleshooting and fix the error code P256A:

  1. Read all data and stored error codes, using an OBD-II scanner.
  2. Clear the codes and check if error code P256A appears again.
  3. Monitor the engine at idle, with gear engaged and disengaged. To check if the engine idle speed matches the values specified in the manufacturer’s technical documentation.
  4. Check the engine for vacuum leakage.
  5. Test the sensor and replace if necessary.
  6. Inspect the throttle for carbon buildup. Also, at the body and inlet of the idle air bypass control valve.
  7. Disconnect and inspect the idle air inlet control valve.
  8. Check that the passages of the idle air control valve are not clogged.

Diagnose and repair of problems

Connect the scan tool and read and write down all available codes. This information may be useful if an intermittent fault is later diagnosed.

If there are other codes present along with the P256A code, diagnose and resolve them in the order in which they have been stored. The most likely additional codes are usually related to the vacuum system.

Therefore, check the vacuum system for leaks caused by damaged vacuum lines. If necessary, repair and remove any codes before operating the vehicle. Then, retest the system to see if any codes return.

Throttle valve

If the code does not disappear, remove the throttle body from the intake manifold and check it for excessive carbon buildup. In some cases, it is possible to remove light carbon deposits from the throttle body and auxiliary air passages using approved cleaners.

However, heavy deposits usually cannot be removed without damaging the throttle body or idle control valve. In these cases, the best option is to replace the throttle body or idle control valve with genuine parts.

When installing cleaned or replacement parts, make sure that all seals and gaskets are also replaced to prevent vacuum leakage. Double check all vacuum line connections before starting the engine or operating the vehicle.

Check wiring and connections

If the P256A code has not disappeared, conduct a thorough visual inspection of all associated wiring. Look for damaged, burned, shorted or corroded wires and connectors.

After no visible damage is found, check the resistance, ground, and reference voltages. Compare all readings with those in the manual. If necessary, replace the wiring to make sure all readings are in accordance with the manufacturer’s specifications.

Stepper motor and idle sensor

Check the actual signal voltage reaching the stepper motor at the connector. Use the scan tool to send a command to the idle speed sensor. Compare the value obtained with the values indicated in the manual.

Special diagnostic programs usually give the voltage range that corresponds to the idle sensor. And sometimes the degrees of rotation of the stepper motor.

These readings must correspond exactly to the stated values, otherwise the harness must be replaced. If all readings obtained agree with the declared values, the stepper motor is defective and must be replaced.

This should solve the problem with error P256A. But if it still appears again, it is possible that the problem is recurring. In this case, diagnosis can be a difficult task. And sometimes you have to let the problem get worse in order to understand exactly where the problem is.

On which vehicles does this problem occur most frequently

Fault code P256A can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Honda
  • Isuzu
  • Jaguar (X-Type)
  • Jeep (Liberty)
  • Subaru (Forester)
  • Toyota

Fault code P256A can sometimes be found with other errors. The most common are the following: P0116, P0118, P0158, P0218, P1028, P1582, P1601, P2100, P256B, P256C, P256D, P256E.


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