A bad electronic control unit often signals the driver by causing the check engine light to remain illuminated. This light comes on for a wide variety of reasons. When an automotive shop is diagnosing problems, its mechanics will generally retrieve the stored codes from the ECU using a scan tool.
If a car suddenly no longer starts, the ECU is sometimes at fault, according to CarsDirect. After checking the normal reasons a car does not start, such as battery and electrical components, the ECU is the next logical component to check.
Poor performance or a drop in fuel efficiency is sometimes due to a faulty ECU. If a driver suspects damage to the unit, the vehicle should be taken to a repair shop. Damaged units often stop working suddenly, leaving the vehicle stranded.
As of Aug. 5, 2012, CarsDirect reports the cost of diagnosis of a defective ECU as $150 to $300. Replacing the unit with a new one costs as much as $3,000, plus another $500 to $600 to install and program the computer. Repairing the unit, if possible, is usually a less-expensive option. With parts and labor, the cost to repair a unit is $300 to $750. Several online companies offer to repair a unit an owner ships to them at a cost of $300 to $400, including reprogramming the unit to work in the car.