A guide to assess control units (draft)
The following is an attempt to create a guide to confirm faulty electronic control units (ECUs) in the field; without the need for a further “third party” tests. This approach is essential to the mobile diagnostic technician like myself; as in many cases, the served garage can choose a cheap and incompetent ECUs testers; giving false conclusions that can lead to a bitter argument and dispute. The word draft is an invitation to all colleagues to contribute in; correcting, adding and improving what is shown in the following three parts:

A) I can see 6 cases of a Bad ECU:  
1) Total loss of Communication; This is the most clear and easy to confirm, once the essential; power, ground and network (CAN, K Line etc.) are all confirmed OK.
But; We have to be careful of having a loss of communication with a good control unit and a satisfying tests of power, ground and network. Here the hidden cause is a bad ECU terminals that can not be visually spotted. This kind of fault is a bit hard to check, but luckily it is a very  uncommon one. One Suggested check method; is to use an Amp clamp around wires and see if this as expected, when the circuit functionality is known.   
 2) Partial and struggle to communicate; provided nothing wrong with the Scanner (Both hardware and software), this can happen sometimes, especially if an incomplete repair was attempted to fix an already faulty ECU. The “Remote” repairer assumes the ECU was fixed, but after receiving and fitting the ECU, only first stages of communication done followed by a failure to give DTCs or live data. Again confirming good; power, ground and network can conclude The ECU is actually faulty.
3) Good communication with a fault of loosing the ECU internal ground to sensors and actuators. In this case giving a temporary external sensor ground can not fix the fault.
Note; For the sake of curiosity; In one faultless ECU job I have checked and confirmed a continuity between the main ECU ground terminals and a sensor ground return (ECU terminal side of course).
4) Good communication with an internal fault of total or partial loss of a reference voltage; The example I have seen is; The ECU got two separate Reference voltage circuits, one is healthy and the other is not.
This subject is a chance to say; Thank you again to Scanner Danner Videos (Both Premium and free) for the brilliant presentation of disconnecting and isolating the sensors form the ECU to be sure that the reference voltage circuit is not pulled down by a bad shorted sensor. What is left is power and ground check to condemn the ECU.  
5) Good communication with an internal fault of a bad actuator driver; commanding coils, injectors and other solenoids. A knowledgable and equipped technician (Scope is needed) can easily and quickly diagnose this type of fault; but the cause of the damage can be sometimes external and in particular faulty ignition coils.
6) An internal fault in a control unit that can completely stop the network communication in the case of high speed CAN, or in the case of Medium/to Low speed CAN the affected network can still communicate with only single wire. A thank you to Positive lead diagnostics You tube Channel for showing a case of; 2007 Audi A6- Comfort Network Issue (single wire mode).


B) An ECU that is affected by Sever electrical Noise: 
Generally speaking Electrical Noise is of 2 types:
1) External (Radiated) Noise from unwanted Electromagnetic Fields. This can be caused by: a) Outside sources; Cases of a Mobile Communication Network Masts and Aerials that can disturb the vehicle security system causing the decarbonisation of the ignition Key.
b) Internally caused by a Magnetic Field emitted from Actuators; Mainly the Ignition system.


2) Internal (Conducted) Noise from the Vehicle components such as:
a) Ignition Coils and spark plugs.
b) Solenoids.
c) Integrated Electronic Circuits.
d) Switches and Sensors; A thank you to South Main Auto Repair You Tube Channel for an example of power steering pressure sensor disturbing an ECU Reference Voltage.
e) Motors (Bad Brushes); A thank you to both Pine Hallow Auto Diagnostics and New Level Auto You Tube for a team work on a case showing a Radiator Fan Motor causing ECU signal disturbance.

C) Cases of Faulty Electronic Power Distribution type ECUs:
Some modern fuse boxes are equipped with an Integrated/or Built in Relays and if one or more of those found faulty; then the whole unit must be replaced. If this fuse box is part of the Body control unit, then this can be classified as a Faulty Control unit with a possible need to do a Programming/or Calibration.

Bashar Alqasim
Trading as Maestro Auto Diagnostics