P1663 — Fuel Demand Command Signal Circuit Malfunction

Diagnostic algorithm

1. Retrieve DTCs and freeze frame; note operating conditions when fault set.
2. Check battery voltage (key ON and cranking) and verify good grounds to ECM and fuel module.
3. Visually inspect connectors and harness along the command circuit for corrosion, pin damage, chafing or water entry.
4. With ignition ON, backprobe the Fuel Demand Command signal at the fuel pump control module and observe with a digital multimeter or oscilloscope. Look for expected voltage/PWM and response to key ON and crank.
5. If no signal, backprobe at the ECM connector to determine whether the signal is lost at the module or at the ECM. If present at ECM but not at module, suspect wiring or intermediate connector.
6. Perform continuity and resistance tests between ECM and module connector pins with power removed. Check for short to battery, short to ground, or opens.
7. Wiggle harness while monitoring signal to identify intermittent faults. Inspect shielding and grounds.
8. If wiring and connectors are good but signal parameters are incorrect, disconnect fuel module and measure driver output under load (follow OEM safety procedures). If output remains incorrect, suspect ECM driver failure.
9. If driver output and wiring are good, test/replace the fuel pump control module or fuel pump driver. Confirm fuel pressure and pump operation.
10. After repair, clear codes, perform road/fuel pressure test under various conditions and re-check for return of fault.
11. If ECM replacement is considered, verify all wiring and components are known-good; follow OEM programming and immobilizer procedures.