SPN 629 - FMI 12 (Fault Code 343)

SPN 629 - FMI 12 (Fault Code: 343)
SPN629 FMI12
ISB, ISC, and ISL with CM2150 - Unswitched Battery Supply

Fault Code: 343
PID(P), SID(S): S254
SPN: 629
FMI: 12/12

REASON: Electronic Control Module Warning Internal Hardware Failure - Bad Intelligent Device or Component. ECM power supply errors have been detected.

EFFECT: Possible NO noticeable performance effects or engine dying or hard starting. Fault information, trip information, and maintenance monitor data can be inaccurate.

Circuit Description:
The electronic control module (ECM) receives constant voltage from the batteries through the unswitched battery wires that are connected directly to the positive (+) battery post. The ECM receives switched battery input through the vehicle keyswitch wire when the vehicle keyswitch is turned on.

Component Location:
The ECM is connected to the battery by the OEM power harness through the ECM battery supply stub. This provides a constant power supply for the ECM. The location of the battery will vary with the OEM.

Conditions for Running the Diagnostics:
This diagnostic runs continuously when the keyswitch is in the ON position.

Conditions for Setting the Fault Codes:
This fault code is triggered when the internal ECM diagnostics detect a read or write error internal to the module.

Action Taken When the Fault Code is Active:
• The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.

Conditions for Clearing the Fault Code:
The ECM will turn off the amber CHECK ENGINE light immediately after the diagnostic runs and passes.

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. If necessary, recalibrate the ECM.

Make sure the ECM unswitched battery supply is coming directly from the battery and not the starter. If unswitched power is coming from the starter, it is possible for the battery voltage to drop low enough during cranking to set this fault active.

This fault can also be caused by resistance in the ECM battery supply (+) or (-) circuits. Resistance in these circuits can cause the voltage level at the ECM input to drop low enough to set Fault Code 343 active.

Read More:
SPN 110 - FMI 2 (Fault Code 334)
SPN 627 - FMI 12 (Fault Code 351)


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