Code
P1621
PEUGEOT
P — Powertrain
Inside ECU (CAN)
AI status
Completed
Completed
100%
Causes
- Faulty inside/interior ECU (software or hardware)
- Loss of CAN bus communication (open, short, high resistance)
- Poor battery or supply voltage to the ECU (low or intermittent)
- Bad ground or corroded connectors at the ECU or junctions
- Blown fuse or poor power distribution to the module
- ECU configuration/coding mismatch after replacement or software update
Symptoms
- Malfunction or loss of interior functions controlled by the inside ECU (e.g., interior lighting, convenience features, central locking)
- Warning or message on dash about communication fault or ECU error
- Intermittent operation of CAN-linked systems
- Some modules not responding to the diagnostic tool or appearing offline
- Possible battery/charging warnings if supply affected
What to check
- Read all stored and pending DTCs on all modules with a capable scan tool; note freeze-frame and event counters
- Check for related U-codes (CAN BUS communication lost) in other modules
- Inspect and verify battery state of charge and terminal connections; check charging system voltage (12.4–14.8 V expected with engine running)
- Visually inspect connectors and wiring at the inside ECU and along CAN trunk for corrosion, damage or water ingress
- Check relevant fuses and relays for the interior ECU power supply
- Measure CAN_L and CAN_H idle voltages at the ECU connector (approx. 2.5 V each; differential ~0 V idle; typical differential when active ~1–3 V)
Signal parameters
- CAN_H voltage (idle ≈ 2.5 V, active pulses to ≈ 3.5 V)
- CAN_L voltage (idle ≈ 2.5 V, active pulses down to ≈ 1.5 V)
- Differential CAN voltage during bus activity (~1.0–2.0 V)
- Bus termination resistance across CAN_H/CAN_L (≈ 60 Ω)
- Battery voltage at ECU connector (key on and engine running)
- ECU supply and ground continuity/resistance
Diagnostic algorithm
- Connect a full-function diagnostic tool and read/record all fault codes across modules; note time stamps and related U-codes.
- Verify battery voltage and charging system; ensure stable supply (repair battery/charging faults first).
- Inspect fuses, relays and the interior ECU connector for corrosion, bent pins or water ingress; reseat connectors.
- With ignition on, measure CAN_H/CAN_L voltages at the ECU connector and at another known-good module location; compare results.
- Check termination resistance across CAN_H and CAN_L (expect ~60 Ω). If open or low, isolate sections by disconnecting branches to find fault.
- Wiggle-test wiring and harnesses while monitoring CAN traffic and communication to reproduce intermittent faults.
- If wiring and power/ground are good but communication remains faulty, attempt to re-establish communication by performing a software reflash/update or ECU reset per manufacturer procedure.
- If reflash fails or ECU reports internal errors, consider module exchange: verify part number and coding, program correctly, then retest. Replace ECU only after confirming wiring and supply are correct.
- Clear codes and test drive; verify fault does not return and that affected functions operate normally.
Likely causes
- Loose or corroded connector pins at the interior ECU
- Damaged CAN high/low wiring harness (pinched, chafed)
- Battery voltage drop during key-on or cranking
- Failed ECU power regulator or internal component
- Recent module replacement or unsuccessful software update leading to corrupted calibration
Fault status
Status
Internal inside-ECU communication fault detected on CAN bus. Fault stored in ECU memory (loss/corruption of CAN messages or internal ECU failure).
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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