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Cummins ISX CM870 (2003-06) Fault Code: 434 PID: S251 SPN: 627 FMI: 2/2 Power Lost Without Ignition OFF. Supply voltage to the ECM fell below 6.2 volts momentarily, or the ECM was not allowed to power down correctly (retain battery voltage for 30 seconds after key OFF).

This fault is set active if the ECM battery supply voltage drops below 6.2 volts while the keyswitch is in the ON position.

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 434 active.

Possible repairs:

1)Tighten the connections.
2)Charge or replace the battery.
3)Repair or replace the OEM power harness.
4)Install the fuse correctly.
5)Repair or replace the ECM power harness.

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