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How to diagnose a faulty control unit without communication

Model: Seat Leon MK 3, 5F - Years 2012-2019
Part: Control unit

Let's see how to diagnose a faulty ECU without communication: if the ECU is not receiving power correctly, it cannot turn on and communicate. We follow this logical sequence: the ECU needs three power supplies: ground. We check that the ground cables are properly connected to the chassis and the engine, and with a multimeter we verify the continuity between the ECU ground pins and a known ground. Second power supply: the permanent one (B+): a cable always under voltage (even with the key off) for memory. We check the voltage, which should be about 12.6V, directly on the ECU PIN. If it is absent, we check fuses and wiring. The third is the cable that receives power when the key is turned. There should be 12V on the corresponding ECU PIN with the key on ON. If it is absent, we check the ignition relay, the fuse, and the wiring. We then verify the ECU outputs. If the power supplies are OK, the ECU should "wake up". Its first critical output is the command to the main relay: the ECU commands this relay by providing a switched ground, so with the key inserted, a specific ECU PIN should switch from 12V to 0V activating the relay. If this signal is missing, it is a strong indication of an internal fault in the ECU. We then check the CAN BUS network using a 2-channel oscilloscope on two CAN bus wires (high and low): we should see two waveforms that are mirror images of each other. If there is no CAN signal, the ECU may be faulty.

Posted on 11 November 2025 by ScegliAuto

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