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Home / DTC / P1668 — PCM IDM Communications Error

P1668 — PCM IDM Communications Error

Detailed page for trouble code P1668.

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Code

P1668

LINCOLN P — Powertrain

PCM IDM Communications Error

Brand: LINCOLN
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Damaged, corroded or loose connector(s) at PCM or IDM
  • Broken, shorted or high-resistance communication wire(s)
  • Blown fuse or loss of power/ignition feed to IDM or PCM
  • Poor or missing ground(s)
  • Defective IDM (ignition driver/module)
  • Defective PCM or internal module failure

Symptoms

  • Malfunction Indicator Lamp (MIL) illuminated
  • No-start or extended crank with no spark
  • Rough idle, misfires or poor engine performance
  • Intermittent engine stall or loss of driveability
  • Other ignition-related DTCs stored or multiple modules not responding on scan tool

What to check

  • Read all stored DTCs and freeze frame data with a capable scan tool; note related codes and history
  • Check battery voltage and cranking voltage (good battery/charging required for reliable comm checks)
  • Visually inspect connectors and wiring at the IDM and PCM for corrosion, bent pins, heat or damage
  • Verify fuses and power feeds to PCM and IDM are present with key ON/crank
  • Verify engine ground(s) and IDC/PCM grounds are clean and secure
  • Confirm presence/response of the IDM on the scan tool or module list

Signal parameters

  • Supply voltage to IDM (battery/ignition feed) — should be ~battery voltage with key ON/crank
  • Ground continuity/resistance for IDM and PCM ground circuits
  • Communication line activity between PCM and IDM (bus/serial pulses or CAN traffic depending on vehicle) — presence/absence and waveform integrity
  • Crank and run data: RPM, ignition coil driver status, misfire counts
  • Module response: ability of scan tool to communicate with IDM and PCM PIDs

Diagnostic algorithm

  1. Retrieve all codes and freeze frame; record OEM-specific PID values and module presence.
  2. Verify battery condition and charging system; low voltage can cause communication faults.
  3. Visually inspect wiring and connectors at the PCM and IDM; disconnect, inspect pins for corrosion, push-tabs, and damage.
  4. Check fuses and measure supply voltage to the IDM with key ON and during crank; repair any open circuits.
  5. Verify ground integrity: measure resistance to chassis/battery negative; clean/repair grounds as needed.
  6. With a backprobe or breakout adapter, check communication line(s) for activity using a scanner/oscilloscope—look for correct waveform or CAN traffic; look for short-to-ground or short-to-B+ conditions.
  7. If wiring and power/grounds are good but comms remain absent or intermittent, substitute a known-good IDM (if available) or test IDM on a known-good vehicle per OEM procedure.
  8. If IDM tests good or substitution unavailable, consult OEM service information for PCM programming updates or reflash procedures and perform programming if required.
  9. Clear codes and perform a functional test/road test; monitor for code return and verify repair.
  10. If code persists after wiring and module checks, escalate to PCM replacement only after confirming programming and manufacturer guidance.

Likely causes

  • Corroded/loose connector at IDM or PCM
  • Open or short in the dedicated communication harness between PCM and IDM
  • Loss of supply voltage or ground to the IDM
  • Faulty IDM module
  • PCM internal fault or programming issue

Fault status

⚠️ Status
PCM detected loss or invalid communications from the Ignition Driver Module (IDM). The PCM has set a P1668 code when expected messages from the IDM are not received or are out of valid range (intermittent or permanent).
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U0100 U0121 P0606 P0601 U0101
166

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