Code
P1655
FIAT
P — Powertrain
Control module malfunction
AI status
Completed
Completed
100%
Causes
- Corrupted or failed module software/firmware
- Intermittent or low power supply to the module (battery, fuse, relay)
- Poor ground or high resistance ground connection
- Damaged wiring or poor connector contact to the module
- CAN/LIN/serial communication bus fault or short
- Internal hardware failure of the control module (processor, memory)
Symptoms
- Malfunction Indicator Lamp (MIL) or warning lamp illuminated
- Loss or intermittent loss of functions controlled by the affected module (locking, lights, engine management, etc.)
- Communication errors shown by the diagnostic scanner or inability to read certain modules
- Intermittent faults that may clear after ignition cycle or battery disconnect
- Vehicles may go into limp/safe mode or refuse to start depending on which module is affected
What to check
- Connect a diagnostic scanner; read and record all stored codes and freeze‑frame data
- Verify battery state of charge and system voltage (engine off and cranking)
- Check for additional U‑codes (communication codes) that indicate bus problems
- Visually inspect fuses, relays and wiring to the suspect control module for damage or corrosion
- Inspect and reseat module connectors; check for bent/pushed pins and signs of water ingress
- Verify key ground points for low resistance to chassis (use DVM, compare to known good ground)
Signal parameters
- Module supply voltage (engine off): ~12.0–12.8 V; (running): ~13.5–14.8 V — verify stable within range
- CAN bus idle voltages: approx. 2.5 V per line (CAN_H/CAN_L) — dominant state shifts to ~3.5 V/~1.5 V respectively
- Ground resistance at module: ideally
- Quiescent module current: normally low (varies by module); abnormally high current may indicate internal fault (refer to OEM spec)
- CAN message activity: consistent periodic messages from other nodes; complete silence or bus errors indicate network issue
Diagnostic algorithm
- Read and document all DTCs, freeze frame and live data from the vehicle using OEM‑level diagnostic tool. Note any related U‑codes.
- Confirm battery voltage and health. Recharge or replace battery if voltage is low or unstable. Recheck code after stable supply is restored.
- Inspect fuses/relays powering the module; measure for presence of battery voltage at module power pin(s) with ignition ON.
- Visually inspect module connector(s) for corrosion, bent pins, water ingress or damage; disconnect and reconnect to clean contacts.
- Check primary ground(s) for continuity to chassis and battery negative; clean and secure ground points if high resistance is found.
- Verify communication on the data bus: use scanner or oscilloscope to confirm CAN/LIN signals and message activity. Repair wiring or shorts if bus is compromised.
- If wiring, power and ground are correct and communications healthy, update or reflash module software per OEM procedures (use dealer/OEM tool where required).
- If reflash/ECU init procedures fail or code persists, substitute with a known good module (or bench test module) if available for confirmation.
- If substitute module clears faults, program and code replacement module per OEM procedures. If not, escalate to module repair or OEM support.
- After any repair, clear codes and perform a full road test and re‑scan to confirm the fault is resolved.
Likely causes
- Low or unstable battery voltage to the module
- Blown or corroded fuse or faulty power relay feeding the module
- Loose, corroded or pushed‑out connector pins at the module
- High resistance ground at chassis/engine ground point
- Open or short on the CAN or local network wiring
- Software corruption requiring reflash or calibration reset
Fault status
Status
Control module malfunction detected (P1655). The module has reported an internal or communication fault. Some vehicle functions may be limited. Further diagnosis of power, grounds, wiring and module software is required.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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