DTC P1001: Data Link Connector (DLC) Malfunction: Condition

WARNING: This page is about a different car, the 2002 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.

DTC P1001 is set when DTCs are not accessible, KOEO or KOER self-tests can not be activated, communication with scan tool problem is present, DTCs displayed by scan tool cannot be listed, or DTC 1001 KOEO self-test cannot be completed. Possible causes are:

DLC is damaged.
Incorrect self-test procedure. DLC is damaged.
Voltage reference is not within specification.
Idle speed is out of specification.
Open or short circuit.
Damaged PCM power relay circuit.
PCM malfunction.