Chart A-2 Won't Flash DTC 12 No Serial Data MIL On Steady

No Scan Data Or Will Not Flash MIL Or MIL (Service Engine Soon) "ON" Steady:

No Scan Data Or Will not Flash MIL Or MIL 9Service Engine Soon) "On" Steady:

CIRCUIT DESCRIPTION
There should always be a steady Malfunction Indicator Lamp (MIL) when the ignition is "ON" and engine "OFF." Battery ignition voltage is supplied to the lamp. The Powertrain Control Module (PCM) will turn the lamp "ON" by grounding CKT 419.
With the "diagnostic" terminal grounded, the lamp should flash a Diagnostic Trouble Code (DTC) 12, followed by any DTC stored in memory.
A steady lamp indicates a short to ground in the lamp control CKT 419 or an open in diagnostic CKT 451, or no engine speed signal on the diesel.

CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. If there is a problem with the PCM that causes a scan tool not to read serial data, then the PCM should not flash a MIL. If MIL does flash, check that the scan tool is functioning properly on another vehicle. If the scan is functioning properly, and CKT 1061 is OK, the PROM or PCM may be at fault for the NO Data Link Connector (DLC) symptom.
2. If the light goes "OFF" when the PCM connector is disconnected, then CKT 419 is not shorted to ground.
3. This step will check for an open diagnostic CKT 451.
4. At this point, the MIL wiring is OK. The problem is a faulty PROM. If MIL does not flash, the PCM should be replaced using the original PROM. Replace the Programmable Read Only Memory (PROM) only after trying a PCM. A defective PROM usually is an unlikely cause of the problem.