Technical ECU bad? BCM lockout triggered?

[TarzanApeman]

Greetings all! Bought my teen daughter a 2019 500L Multi-Aire 1.4. It was pretty cool for three weeks & then..
Was working on the brakes & unlocked the door with the physical key, alarm went off, deactivated it with the physical key in the ignition. After got done with brakes - it wouldn't start. Starter doesn't turn over. Battery good. ALL service warnings come up on dash display- including Security Locked message. I looked at the ECU & the previous owner had pried it open for some reason. Again, it ran fine for three weeks, though! Unplugged the ECU & the behavior is still the same. (ALL service warnings) It's almost like the ECU doesn't exist. Took the car to a friend's shop. Hooked it up to a professional SnapOn scan tool & it couldn't communicate w/ the ECU. Mind you, they are not a Fiat shop & don't have a Fiat subscription. I read that the BCM could be causing the ECU from being seen? (cause of the security lockout?) Does this make sense? The BCM is far cheaper than replacing/reprogramming the ECU. Also just read the ECU has its own fuse? (I didn't think to look for that ) BTW - the SGM was removed & replaced with the bypass connector. THANK YOU ALL IN ADVANCE!

 

[Mike1alike]

The symptom pattern you’re describing is classic no-ECU-communication/immobiliser lockout, not a random brake or alarm fault.

Key points from what you wrote:
All service warnings at once
“Security Locked” message
Starter does not crank
Snap-On tool can’t communicate with the ECU
Unplugging the ECU changes nothing
ECU casing has been pried open.

That combination strongly points to the ECU not powering up or not being seen on the CAN bus, rather than a BCM failure.

A bad BCM can cause immobiliser lockouts, but it would not normally make the ECU completely invisible to a diagnostic tool. When a Fiat ECU can’t be communicated with at all, the usual causes are:
* No ECU power or ground (blown ECU fuse, relay, corroded connector, damaged wiring)
* ECU internal failure (very likely here, given it’s been pried open)
* CAN bus line open/shorted at the ECU connector.

Before touching the BCM check all ECU fuses and relays (yes, the ECU does have its own feeds), verify 12 V and ground are actually present at the ECU connector and check CAN-H/CAN-L continuity at the ECU plug. If the ECU has power and ground but still won’t talk to a scan tool, it’s almost certainly dead internally.
The alarm incident was likely just the trigger, not the root cause. The ECU was probably already compromised from being opened.
Also worth noting: a generic Snap-On tool plus no Fiat software subscription won’t help much here.
A Multiecuscan or dealer-level tool is needed to see BCM/immobiliser status properly, but even that won’t fix a dead ECU.
Bottom line: don’t buy a BCM yet. Prove ECU power/ground/CAN first. Given the tampered ECU case and total loss of comms, the ECU itself is the prime suspect.

 

[Mike1alike]

We: “That complicates things…”
Fiat engineers: “Hold my CAN bus.”

 

[bugsymike]

It is a bit concerning that previous owner had tampered with the ECU.
From my small understanding the correct chipped key talks to the antenae ring around the ignition switch which in turn tells the BCM it is unlocked and for the ECU to permit starting of engine.
I use a "break out box" to plug into the OBD port allowing testing without damage, this can show both the live and earthing points related to the ECU and allow further testing of all 16 pins.
Can OP go back to previous owner as presumably having tampered with the ECU he will have some knowledge of what the issue is.
Some time ago I had to fit a new BCM to a Fiat Punto Evo after previous owner had shorted a seat occupancy sensor when trying to repair it. I was able to use a friends up to date Snap On Zeus tool to allow it to identify all components correctly and car has proved reliable for over four years since then. Obviously a s/h BCM will run into immobiliser issues in not recognising the chipped key, where as the brand new genuine Fiat was able to start from fresh.