P1620

Code

SCION P — Powertrain

ECM communication circuit malfunction

Brand: SCION

Type: P — Powertrain

Views: UK: 2 EN: 6 RU: 6

AI status

Completed

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Page language: EN

Causes

Blown fuse or faulty ECM power/ignition feed
Poor or corroded ground(s) to the ECM
Damaged, shorted, or disconnected wiring/connectors between ECM and other modules (CAN, LIN, serial)
Faulty ECM or other control module (immobilizer, BCM)
Network bus (CAN/LIN) termination fault or short to voltage/ground
Low battery voltage or intermittent supply during cranking

Symptoms

Check Engine MIL or security/immobilizer lamp illuminated
No-crank/no-start or intermittent starting problems
Loss of functions controlled by ECM or other modules
Intermittent faults or multiple communication-related DTCs present
Accessory or ECU-linked features inoperative

What to check

Retrieve all stored/related codes with a capable scan tool and note freeze frame
Verify battery voltage (12.4–12.9 V resting, >9.6 V during cranking) and charging system
Inspect ECM fuses and relays related to power and communication
Visually inspect ECM and related module connectors for corrosion, bent pins, or water intrusion
Check main engine grounds and chassis grounds for tightness and corrosion
Use scan tool to confirm whether ECM responds and whether other modules appear on the network

Signal parameters

Battery voltage at ECM power pin (should be ~12 V with key ON, >9.6 V while cranking)
Ignition-switched power at ECM IGN feed
CAN High voltage ~2.5 V nominal (idle) and CAN Low ~2.5 V nominal; differential waveform present during activity
Bus resistance between CAN H and CAN L (approx. 60 ohms with two 120Ω terminators)
Presence/absence of expected module IDs/messages on scan tool (ECM and immobilizer/BCM responding)

Diagnostic algorithm

Read and record P1620 plus any U-codes or related DTCs. Clear codes and attempt to reproduce.
Verify battery state and charging; recharge or replace battery if low and retest.
Inspect ECM and module fuses; replace any blown fuses and test for recurring faults.
Visually inspect and wiggle-test connectors, wiring harnesses, and grounds for intermittent faults; repair obvious damage.
Using a scan tool, verify whether ECM communicates. If ECM is non-responsive while other modules are present, suspect power/ground or ECM fault.
Check power and ground pins at ECM connector with a multimeter. Repair poor connections or wiring faults.
Test CAN/LIN bus: measure voltages on CAN H/L, check for correct idle voltages and differential waveform with oscilloscope if available; isolate short by disconnecting segments or modules.
Disconnect aftermarket devices that may interfere and retest communication.
If wiring and power/ground are confirmed good but communication fails, test or swap the suspect remote module (immobilizer/BCM) if possible or consult manufacturer procedures for module substitution.
If ECM is suspected, follow OEM steps for reprogramming or replacement; after repair, clear codes and verify repair by confirming stable communications and no reoccurrence.

Likely causes

Loose/poor ground or power supply to ECM
Open/short in communication wiring (CAN high/low or serial lines)
Faulty connector at ECM or related module (corrosion, bent pins)
Bad ECM or failed immobilizer/BCM module
Blown communication or ECM fuse

Fault status

⚠️ Status

ECM communication circuit malfunction detected. Communication between ECM and other modules interrupted or out of specification; MIL/immobilizer lamp may be on.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-3.0 hours

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P1620 — ECM communication circuit malfunction