Code
P1668
MERCURY
P — Powertrain
PCM IDM Communications Error
Views:
UK: 19
EN: 65
RU: 25
AI status
Completed
Completed
100%
Causes
- Open, short, or high resistance in wiring between PCM and IDM
- Poor or corroded connectors or pins at PCM or IDM
- Blown fuse or loss of battery power to IDM or PCM
- Bad IDM (ignition driver module) or failing PCM
- Faulty grounds at engine or chassis affecting IDM/PCM
- Intermittent faults caused by heat, vibration, or damaged insulation
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Hard starting or no-start
- Misfire or rough running
- Loss of ignition to one or more coils
- Intermittent engine stalling or loss of power
- Related DTCs for ignition or communication circuits
What to check
- Scan for stored and pending codes and retrieve freeze frame data
- Verify battery voltage (should be ~12.6 V key off, >11 V cranking) and charging system
- Check fuses and ignition relay feeding PCM and IDM
- Visually inspect connectors at PCM and IDM for corrosion, bent pins, or water intrusion
- Wiggle wiring harness with engine running to try to reproduce fault
- Measure continuity and resistance of wiring between PCM and IDM (with power removed)
Signal parameters
- Power supply to IDM: battery voltage (~12 V) with key ON/run
- IDM ground: near 0 ohms to chassis
- PCM command/comm line: switching pulses (0–12 V) or low-side driver pulses synchronized to crank/trigger; frequency proportional to engine speed
- No unexpected open-circuit or short to battery/ground on communication/command circuits
- Coil primary waveform: pulses present when PCM commands ignition
Diagnostic algorithm
- Connect a professional scan tool and confirm P1668 and any related codes; record freeze frame and clear codes.
- Verify battery and charging system health; low voltage can cause communication faults.
- Inspect connectors and wiring at the PCM and IDM for corrosion, bent pins, water intrusion, or melted insulation.
- Check fuses and power/ignition feeds to IDM and PCM; replace any blown fuses and re-test.
- Backprobe the IDM power and ground with key ON to confirm proper supply and ground; repair as required.
- With key ON and engine cranking (if safe), use a digital scope or DVOM to check the PCM-to-IDM command/comm circuit for expected switching pulses; note waveform, amplitude, and frequency.
- Check continuity/resistance between PCM pin and IDM pin for the communication/command circuit (power off). Repair any opens or high resistance.
- Wiggle test harness while monitoring for intermittent changes. Repair any chafed or broken wires and retest.
- If wiring and supplies are verified good and connector pins are clean, substitute a known-good IDM (if available) and re-check. If fault persists, consider PCM testing or replacement after confirming PCM outputs and inputs.
- After repairs, clear DTCs and perform a road or bench test to confirm the fault does not return. Document findings.
Likely causes
- Damaged wiring or connector between PCM and IDM (most common)
- Corroded or loose ground or power supply to IDM
- Failed IDM module
- PCM internal fault or corrupted software (less common)
Fault status
Status
PCM detected loss or improper communication with the Ignition Driver Module (IDM).
Repair difficulty: Medium
Diagnostic time: 1.5-4.0 hours
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