SPN 729 - FMI 4 (Fault Code 2556)

Intake Air Heater 1 Circuit - Voltage Below Normal or Shorted to Low Source

ISB, ISC, and ISL - Intake Air Heater 1 Circuit

Fault Code: 2556
PID(P), SID(S): S070
SPN: 729
FMI: 4
Lamp: Amber

REASON: Intake Air Heater 1 Circuit - Voltage Below Normal or Shorted to Low Source. Low voltage detected at the intake air heater signal circuit.

EFFECT: The intake air heater may be ON or OFF all the time.

Circuit Description:
The intake air heater improves starting and white smoke control in cold ambient conditions. The electronic control module (ECM) controls relays that switch power to the intake air heater. 12-volt systems will have two relays while 24-volt systems only require one relay.

Component Location:
The intake air heater is located at the air inlet connection into the intake manifold. The location of the intake air heater relays will vary by original equipment manufacturer (OEM).

Conditions for Running the Diagnostics:
This diagnostic runs when the keyswitch is in the ON position and when the pulse width modulated (PWM) device is getting energized or de-energized. In some cases the diagnostics can also run at some fixed intervals.

Conditions for Setting the Fault Codes:
The intake air heater circuit PWM signal is not detected to be system voltage when the PWM signal is turned on.

Action Taken When the Fault Code is Active:
• The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.
• The intake air heater circuit will be disabled.

Conditions for Clearing the Fault Code:
The ECM will turn off the amber CHECK ENGINE light and the fault code state will become inactive immediately after the diagnostic runs and passes. The fault code can also be cleared with INSITE™ electronic service tool.

Shop Talk:
Verify the electronic control module (ECM) calibration is correct. Check the calibration revision history found on QuickServe® Online for applicable fixes to the calibration stored in the ECM.
Possible causes of this fault code include:
• A shorted intake air heater relay
• Intake air heater relay supply wire shorted to ground.

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