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as you're not able to get communication with the ECU then I'd start by checking the ECU fuses for damage / cracks etc and then use a meter to see if there's 12V at the fuse with the ignition on
you don't say what model / engine size this is but there should also be an ECU power relay in the fusebox under the bonnet
if there's power at the fuse check to see that this relay is clicking under your touch when the ignition is flipped on
if there's 12V on the fuses then you're right in saying its not the immobilser
but it sounds likely that the ECU isn't opening the fuel injectors, if the injectors are working you can hear them click - its difficult to hear them with background noise so use a long screwdriver like a stethoscope, placing the handle end in your ear and the metal tip against the body of the injector whilst your assistant turns the engine over.
If you can't hear anything check the ECU power is present by measuring voltage at the fusebox and on its relay socket if OK then check the connections to the crank speed sensor and consider replacing this sensor if the wiring / connections look OK
If the car will start using a squirt of quick/ easi start into the intake then this confirms the problem is lack of fuel
OK if there's ECU power then the next step is to check the state of the small earth wires that run out of the engine bay wiring and are secured to the engine with small bolts and ring terminals
these wires are the ECU earth connections
if this is OK then I'd say the next step is to get the unit sent off for testing
do you still need help?
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