Reason:
Engine Control Module Warning Internal Hardware Failure – Bad Intelligent Device or Component. ECM power supply errors have been detected.

Effect:
Possible reduced engine performance.

Circuit Description:
The engine 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 harness. This direct link provides a constant power supply for the ECM. The location of the battery will vary with the original equipment manufacturer (OEM). Refer to the OEM service manual.

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

Conditions for Setting the Fault Codes:
The Engine Control Module (ECM) detected a read or write error internal to the ECM.

Action Taken When the Fault Code is Active:
The ECM illuminates the amber CHECK ENGINE lamp and/or the Malfunction Indicator Lamp (MIL) immediately when the diagnostic runs and fails.

Conditions for Clearing the Fault Code:
1) To validate the repair, perform a key cycle, start the engine and let it idle for 1 minute.
2) The fault code status displayed by INSITE™ electronic service tool will change to INACTIVE immediately after the diagnostic runs and passes.
3) The ECM will turn off the amber CHECK ENGINE lamp immediately after the diagnostic runs and passes.
4) For On-Board Diagnostics (OBD) engines, the ECM will extinguish the Malfunction Indicator Lamp (MIL) after three consecutive trips where the diagnostic runs and passes.
5) The Reset All Faults command in INSITE™ electronic service tool can be used to clear active and inactive faults, as well as extinguish the MIL for OBD applications.

Shop Talk:
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.

Possible Causes:
1) Faulty batteries
2) Faulty power connector
3) Damaged harness
4) Damaged ECM connector pins
5) Open circuit in the battery power circuit
6) Incorrectly installed OEM fuse
7) Blown OEM fuse
8) Faulty ECM