2.0 Diesel engine ECU

[ag_cohen113]

my car had an intermittent problem with the engine cutting out but would always restart, the last time this happened when it came to restarting the starter would not work. the starter was replaced with a known good one and still the same issue. I have checked all the fuses and had it plugged in for diagnosis however the snap on diagnosis will not communicate with the engine ECU which has lead me to either a wiring fault or an ECU issue. my problem is I don't know where the ECU is, can anybody help with this please and has anybody come across this before?
Thank