Title: Seeking Advice: Car Stalls, No Start, Security Light On (Codes P0300 and Anti-Lock System) - Possible ECU Issue?

Hello, fellow Redditors! I'm currently facing a frustrating issue with my car and could really use your expertise and guidance. Here are the symptoms and the replacements I've made so far:

Symptoms: - Car stalls and fails to start - Security light illuminates - Error codes: P0300 (random/multiple cylinder misfire) and Anti-Lock System

Replacements: - New Fuel Pump - New Alternator - New Spark Plugs and Coils - New Crankshaft Sensor - New Catalytic Converter - New Battery

Details: Before my car completely stopped sparking, I had a mechanic replace the alternator while the battery post was still connected, resulting in a spark. Since then, I've been unable to start the car. Interestingly, the dash lights still come on, which suggests that the issue might not be related to the positive terminal. Therefore, I suspect that the ECU (Engine Control Unit) might be at fault.

I've attempted to test the ECU fuse, but I'm not entirely confident in my approach and the results. It appeared to be fine, but there might be other factors I'm overlooking.

Now, I would greatly appreciate any advice or suggestions you can provide to help me troubleshoot and potentially resolve this issue. Specifically, I have a few questions:

1. Could the spark during the alternator replacement have caused damage to the ECU or other components?
2. Are there any specific steps or techniques to properly test the ECU fuse or diagnose potential ECU issues?
3. Are there any other components or connections that I should check for damage or malfunction that could cause these symptoms?

While I understand that seeking professional help may ultimately be necessary, I'm hoping to gather some initial insights and possible solutions from the community before taking that step. Your knowledge and experiences would be tremendously helpful in resolving this frustrating situation.

Thank you all in advance for your time and assistance. I'm looking forward to your suggestions and advice!