Code
P1655
ALFA ROMEO
P — Powertrain
Control module malfunction
AI status
Completed
Completed
100%
Causes
- Control module internal hardware or software failure
- Loss of power or ground to the module (blown fuse, poor connector, corroded terminal)
- Faulty or intermittent CAN/LIN bus communication
- Damaged wiring harness or short/open circuits
- Corrosion, water ingress or physical damage to the module or connectors
- Aftermarket electronic devices or incorrect replacement module programming
Symptoms
- Malfunction Indicator Lamp (MIL) or warning light illuminated
- Multiple systems showing faults or loss of function (locks, lights, instrument cluster, engine management, etc.)
- Intermittent or permanent loss of communication with the affected module on a scan tool
- Certain vehicle functions disabled or degraded
- Vehicle may fail to start or have erratic behavior if the module controls critical systems
What to check
- Connect OEM-level scan tool; read and record all stored and pending DTCs and freeze frame data
- Verify battery voltage with ignition on and engine off (should be ~12.4–12.8 V at rest; >13.5 V when charging)
- Check relevant fuses and fusible links for the module
- Inspect module connectors and wiring for corrosion, bent pins, water intrusion, or damage
- Perform wiggle test on harness while monitoring communication and live data for intermittent faults
- Check CAN/LIN bus voltages (see signal_params) and verify proper termination resistance (~60 ohms across CAN H/L at idle)
Signal parameters
- Battery supply to module: ~11–14.8 V depending on state (engine off/on)
- CAN bus idle voltages: CAN_H and CAN_L nominally near mid-supply (~2.5 V); dominant state approx. CAN_H ~3.5 V and CAN_L ~1.5 V
- CAN differential: verify data activity with scan tool or oscilloscope; expected periodic CAN frames when ignition on
- Termination resistance across CAN_H and CAN_L: ~60 ohms (two 120-ohm resistors in parallel)
- Module wake/sleep control lines: follow manufacturer wiring diagram for expected logic levels (battery or ground referenced)
Diagnostic algorithm
- Retrieve and document all DTCs from all control modules and note time of occurrence and freeze frame data
- Check vehicle battery state of charge and charging system; recharge or load-test battery if below spec
- Visually inspect module and connectors for corrosion, moisture, damage; disconnect and reseat connector and inspect pins
- Check and verify fuses and relays related to the module; replace if faulty
- Measure power and ground at the module connector while monitoring DTCs; repair any poor connections or high-resistance grounds
- Probe CAN bus at the module with multimeter or oscilloscope: check idle voltages, differential activity, and termination resistance
- If communications are lost, trace wiring back to harness junctions and other modules; repair short/open faults and clean corroded contacts
- Perform a module reset (clear codes) and cycle ignition; see if faults return. If intermittent, perform wiggle/pressure tests while monitoring
- Check for available software updates or known-service-bulletins; reflash or reprogram module per OEM procedures if recommended
- As a last resort, substitute a known-good programmed module (if available) to verify behavior before replacing; ensure correct part number and immobilizer/co‑programming steps are followed
- Document repairs and confirm no new DTCs and restoration of communication and affected functions
Likely causes
- Poor battery voltage or weak battery causing module resets
- Loose, corroded, or damaged power/ground connections at the module
- Faulty CAN bus wiring or missing termination resistors leading to lost communication
- Corroded connector pins or water-damaged module housing
- Software corruption or need for module reflash/update
Fault status
Status
Control module malfunction — internal fault or communication/power problem detected. Further diagnostics required to identify root cause.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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