Possible causes

– Loose or poor battery terminal connection
– Low battery charge
– ECM power supply circuit is open or shorted
– ECM Programing
– Faulty ECM

Tech notes

Start by charging the vehicles battery and clean terminal, once the battery is charged and the terminals are clean and secure, erase code. If the code comes back ON you may have to reprogram or replaced ECM

When is the code detected?

Indicates the ECM has experienced an internal memory fault.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Engine may not start

P0603 HONDA Description

Battery voltage is supplied to the ECM even when the ignition switch is turned OFF for the ECM memory function of the DTC memory, the air-fuel ratio feedback compensation value memory, the idle air volume learning value memory, etc.

HONDA Technical Service Bulletins for P0603 code