Some effects of a broken engine control unit (ECU) vary among vehicle makes, models and years, but many symptoms are common among different cars. The ECU is expensive, so it's important to accurately diagnose the problem before buying a replacement.
Engine Cranks but Won't Start
The ECU does not normally affect the starter. If the ECU is broken, the engine turns over normally but will not start or show any other signs of life.
Spark May Be Present
Depending on the design, the spark plugs may or may not produce spark if the ECU is broken. A traditional ECU controls the fuel injection system, not the ignition spark. In electronic ignition systems, spark is controlled by a computer that may be integrated into the ECU or another unit.
The Fuel Pump Works
Cranking the engine should trigger the fuel pump even if the ECU is broken. Fuel is still delivered to the injector rail, but the fuel injectors cannot function.
No "Check Engine" Light
Normally, a vehicle's "check engine" light should illuminate when the ignition key is in the "on" position but the engine is not running. Absence of this light could indicate ECU failure.
No Trouble Codes
A code reader tool is designed to interpret "trouble codes" from the ECU. The ECU also produces "proper operation" codes that indicate the controller is working properly. The absence of any codes usually means the ECU is broken.
- Photo Credit "Audi A6 2.5 TDI - Water pump replacement" is Copyrighted by Flickr user: whiskymac (Reg Mckenna) under the Creative Commons Attribution license.