Code
P1656
LINCOLN
P — Powertrain
CAN Link PCM PCM Circuit Network
Views:
UK: 16
EN: 36
RU: 24
AI status
Completed
Completed
100%
Causes
- Damaged or chafed CAN high/low wiring or connectors
- Corroded or loose connector pins at PCM or network modules
- Open or shorted termination resistor(s) (incorrect ~60 Ω across CAN H/L)
- Faulty PCM or another module loading the bus
- Blown fuse or loss of PCM power/ground
- Water intrusion or contamination in connectors
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Intermittent or permanent loss of communication with PCM on a scan tool
- Multiple unrelated modules reporting errors or inoperative functions (ABS, traction, climate, cluster, etc.)
- No crank/no start or abnormal engine operation if PCM cannot communicate
- Intermittent features (cruise, shift control, immobilizer) not functioning
What to check
- Retrieve freeze frame and full scan for additional U/P codes to identify affected networks or modules
- Verify battery voltage is within specification and battery terminals are clean/tight
- Inspect PCM and gateway module connectors for corrosion, bent pins, water, or damage
- Check fuses and power/ground circuits for the PCM
- Measure DC voltages on CAN_H and CAN_L at PCM connector with ignition ON
- Measure termination resistance across CAN_H and CAN_L (should be ~60 Ω total with network connected)
Signal parameters
- CAN bus recessive idle voltage (each line) ≈ 2.5 V (both CAN_H and CAN_L biased to ~2.5 V)
- Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ~2.0 V)
- Termination resistance across CAN_H and CAN_L ≈ 60 Ω (two 120 Ω terminators in parallel)
- Bus data rate (vehicle dependent) commonly 250 kb/s or 500 kb/s — oscilloscope should show encoded waveform at correct baud
- No DC short: neither CAN line should be shorted to battery voltage or chassis ground
Diagnostic algorithm
- Read and record all stored codes and freeze frame data with a capable scan tool. Note other U-codes or related module failures.
- Verify battery state-of-charge and charging system; clear codes and attempt to reproduce. If code returns, continue.
- Visually inspect PCM and network module connectors and wiring harnesses for corrosion, damage, or water intrusion.
- Check PCM power and ground circuits and related fuses/relays. Repair any power/ground issues before continuing.
- With ignition ON, measure DC voltage at the PCM CAN_H and CAN_L pins relative to ground. Expect ~2.5 V idle on each; note any stuck high/low or 0 V.
- Measure resistance across CAN_H and CAN_L (with ignition OFF). Expect ~60 Ω. If open or very low, isolate network and locate missing/shorted terminator or wiring fault.
- Use an oscilloscope to observe CAN differential waveform while cranking/idle. Look for missing frames, noise, no dominant states, or distorted edges.
- Isolate the fault by disconnecting modules one at a time (when safe) and rechecking bus electricals and communication to find the device that causes the fault to clear.
- Repair the failing wiring, connector or replace the faulty module identified. Protect and seal connectors against moisture.
- Clear codes and retest vehicle operation and network communication. Monitor for reoccurrence and verify related systems function normally.
Likely causes
- Poor ground or supply to PCM (check battery/fuse/ignition power)
- Open, short to ground, or short to battery on CAN_H or CAN_L wiring
- Missing or failed termination resistor (open or shorted)
- Corroded/loose PCM connector pins
- Failed PCM or other module pulling the bus low/high
Fault status
Status
CAN Link PCM PCM Circuit Network — PCM detected a fault on the CAN communication network (loss of communication, bus voltage abnormality, or open/short condition). Communication between PCM and network modules may be impaired.
Repair difficulty: Hard
Diagnostic time: 1.5-3.0 hours
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