Engine Problems:
* No Start: The most severe problem – the engine simply won't crank or start.
* Rough Idle: The engine idles erratically, shaking violently.
* Stalling: The engine cuts out unexpectedly, especially at idle or low speeds.
* Misfires: The engine runs roughly, possibly with reduced power, and may throw a check engine light.
* Poor Acceleration: Sluggish acceleration and lack of power.
* Hesitation: The engine hesitates or stutters during acceleration.
* Backfiring: The engine backfires through the exhaust or intake.
* Overheating: While not directly caused by the ECU, a faulty ECU can cause incorrect fuel/air mixture leading to overheating.
* Poor Fuel Economy: Significant drop in gas mileage.
Other Issues:
* Check Engine Light: This will almost certainly be illuminated, often with multiple codes.
* Transmission Problems: The ECU also controls aspects of the automatic transmission (if equipped), so problems like slipping or shifting issues could arise.
* Electrical Issues: Some electrical components controlled by the ECU might malfunction. This isn't always directly attributable, but some strange electrical gremlins could be a symptom.
Important Note: Many of these symptoms aren't *exclusive* to a bad ECU. Other components, like sensors, wiring harnesses, or the engine itself, can also cause similar problems. A proper diagnosis is crucial, usually requiring a mechanic to scan the ECU for diagnostic trouble codes (DTCs) and perform further tests to pinpoint the exact cause. Simply replacing the ECU without a proper diagnosis could be a costly mistake if the underlying issue lies elsewhere.
