How to Diagnose ECU Communication Issues?




Here is what you need to check:


💡 Tip. Before you start chasing the wiring issues

  • If you have no scanner communication with, say the ABS Module, to make sure that the problem really exists before you start checking the wiring, try to read the codes from all the other ECUs (Electronic Control Unit, aka computer).
  • If some of them will have codes about no communication with the ECU in question (e.g. ABS), then you just verified that there is a problem to diagnose.
  • If there are no codes, try a different scan tool first, it can save you a lot of unnecessary diagnostics time. Keep in mind that frequently other ECUs will not show you any codes about communication issues even though one or more ECUs might be disconnected from the network. So don’t fully rely on this test if there are no codes.


Here is an oversimplified wiring diagram example of what wiring you might want to check, before you would condemn the ECM as faulty:

ECM Communication Wiring Troubleshooting



1. Power
  • Usually each ECU has at least one wire that supplies battery power to it
  • ECM (Engine Control Module) usually has one 12V power wire that is powered at all times (B+) and another one that is powered with ignition switch ON (IGN).
  • Also, ECM usually has a Main Relay (aka EFI Relay, DME Relay, etc.) which supplies 12V to all kind of engine sensors and also (frequently but not always) to the ECM.
    • Usually ECM is the one that energizes (grounds) the Main Relay, and when it’s energized,  ECM receives an additional 12V from it. Without this 12V from the Main Relay in a lot of cases ECM will not be awake, and will not communicate with the scanner.
    • Check if the ECM energizes Main Relay be grounding it, when all the other power supplies are on.
      • If Main Relay is not being activated by the ECM, there is a very slight chance that 5V sensor supply line is shorted to ground (directly or through one of the sensors). Finding that kind of short might involve a lot of work, disconnecting all the sensors that are supplied by that 5V and tracing wires for shorts,  corrosion.
      • Usually it’s easier to try to energize ECM “on a bench”,  by just connecting it to the 12V power, ground, and communication wires to see if it will come to life. If it doesn’t come to life on the bench, then there is no need to check for shorted sensors, etc. Simply replace the ECM.
      • 📝 No Start | Shorted Vehicle Speed Sensor | 2002 Acura.
        • Car has died when engaged in reverse (twice). Then wouldn’t restart. When transmission module was disconnected, car started fine. Turned out to be a vehicle speed sensor.
  • Note, all the power wires have to be tested under load.
2. Ground
  • Note, all the ground wires have to be tested under load.
    • 📝 If the car starts with OBDII scanner plugged in, but doesn’t start with scanner unplugged (PCM relays might be clicking), check the ECM ground.
3. Communication wires
  • Most modern vehicles usually utilize protocols like CAN Bus Protocol, K-Line Protocol, LIN Bus Protocol, etc., for communication between computers and with the diagnostic scanner.
  • Read more below.


Communication Network



CAN Bus in cars is standardized in ISO 11898-2 (more details in here)

All CAN Bus pair wires (CAN H and CAN L) are twisted together.

  • The twisted pair wire is an essential part of how the differential mode filtering works on a CAN Bus, and without it the signal can be easily distorted.
Terminating Resistors
  • As you can see on the picture above, CAN Bus wires are usually connected by the two terminating resistors in parallel somewhere on the network, each separately measuring 120Ω.
    • When they are both connected to the network, resistance between CAN Bus wires becomes 60Ω (link).
  • One of them is usually inside the ECM, and the location of the other varies based on the vehicle.
  • Without them there will be a lot of noise on the CAN Bus.
  • If the ECU with the terminating resistor is disconnected, resistance between CAN Bus wires will become 120Ω.
  • If resistance is more than 120Ω, it means that there is a problem with CAN Bus wires and/ or possibly with both ECUs that host terminating resistors.
Resistance Test
  • A simple and a quick CAN Bus check can be done with an ohmmeter.
    • Turn ignition OFF, and measure resistance between terminals 6 and 14 of the OBDII connector.
    • If the ohmmeter shows 60Ω, then you can move on to the next test. CAN Bus termination test passed.
    • If it shows 120Ω, then there is an issue with the circuit for one of the terminating resistors (could be the wiring or the ECU hosting the terminating resistor).
    • If the ohmeter shows much more than 120Ω, suspect open CAN Bus wiring.


Differential voltages

CAN Bus Levels ISO 11898

  • Testing differential voltages will require a good voltmeter with min-max function (e.g. Fluke 88V) or a oscilloscope (e.g. Pico 4424).
  • CAN High usually (ISO 11898-2) ranges between 2.5V to 3.5V, and CAN Low between 1.5V to 2.5V.
  • CAN Bus assigns logical ‘1’ to a typical differential voltage of 0V, and the logical ‘0’  with a typical differential voltage of 2V.
  • The CAN Bus is in the dominant state (logical 1) if the differential voltage is greater than 0.9 V and in the recessive state (logical 0) if it is less than 0.5 V.


📝 Communication Bus | BMW Mini

🩺 No Start | U1000 CAN Communication Circuit | 2006 Nissan Frontier

🩺 CAN Bus Fault | Faulty Module | Audi TT

🩺 No Start, CAN Bus Fault | Bad CAN L wire | Vauxhall Corsa

🩺 Multiple Electrical Issues, B-CAN bus fault | Corrosion | 2007 Honda Odyssey

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