Toyota/Lexus ECU/ECM

Toyota/Lexus ECU/ECM U0100 Post-Repair Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired Toyota/Lexus ECU/ECM and the problem is still happening.

Important: Please do not open/disassemble the module. This can damage the unit and may void warranty coverage.

Step 1: Confirm the correct ECU was installed

• Verify the part number on the ECU matches the original unit from the vehicle
• Confirm the ECU is mounted/bolted correctly (poor mounting can stress connectors)

Step 2: Battery voltage and basic reset

• Battery must be fully charged (low voltage can cause U0100/no-comm)
• Disconnect battery negative for 10 minutes, reconnect, then key ON and recheck

Step 3: ECU connector seating and pin check

• Key OFF
• Unplug ECU connectors and inspect for:
• Bent pins, pushed-back pins, corrosion, moisture/oil intrusion
• Broken lock tabs or connectors not fully latched
• Re-seat each connector until fully locked

Step 4: Check ECU power and grounds (most common cause)

• Confirm ECU-related fuses are good (ECU/EFI/IGN/INJ fuses vary by model)
• Check main ECU relay operation (swap with like relay if possible)
• Inspect and clean battery terminals and main chassis/engine ground points

Step 5: Verify 12V and ground at the ECU connector

• With key ON, a shop should confirm:
• Battery power feed(s) present at ECU
• Ignition-switched power feed(s) present at ECU
• Grounds have low resistance and are not loose/corroded
• If power/ground is missing, the ECU cannot communicate

Step 6: Network (CAN) quick checks

• If multiple modules show “no communication,” suspect a network issue
• Inspect for recent work: battery replacement, jump-start, stereo/remote start install, rodent damage, collision/water intrusion
• Check wiring near the ECU and along the firewall for rub-through

Step 7: Scan tool behavior matters

• If the scan tool cannot connect to ANY modules, start at DLC power/ground (OBD port) and related fuses
• If scan tool connects to some modules but not ECU, focus on ECU fuses/relay/power/ground and ECU CAN wiring

Step 8: If U0100 returns immediately after reinstall

• Double-check ECU connectors for pushed-back pins
• Confirm ECU relay is clicking/energizing with key ON
• Verify battery voltage during crank (weak battery can drop voltage and kill comm)

Step 9: What to send UpFix if the issue continues

• VIN
• Full DTC list from ALL modules (not just ECU)
• Does the scan tool connect to other modules? (Yes/No)
• Any recent electrical work, jump-start, water intrusion, or rodent damage