Why ECUs fail

Engine Control Units (ECUs) fail for these three broad general reasons.

  1. Component degradation due to environmental causes
  2. Insult from other vehicle systems.
  3. Insult from outside of the vehicle.

Insults from outside of the vehicle.

Insulting your ECU by calling it a "son of a Volvo" is begging for trouble, so you should be always be careful of what you say near them. However, considering that the Bosch ECU's are typically German made, we realize they are usually stoic about such trifling words.

The kinds of insults which will kill an ECU are electrical, and these sorts have been reported:

  • Jump starting a Saab with the terminals switched, that is crossing the cables between the two cars from positive to negative and negative to positive.
  • Jump starting using inferior jumper cables. This can allow strong and unstable current to reach the ECU.
  • When a muffler shop uses a welder to replace the Saab muffler, and they neglect to disconnect the ground cable from the battery before energizing the welder, then high voltage (and probably spiky voltage) finds its way to the ECU and fries it.

Insults from other vehicle systems

A number vehicle faults will damage an ECU, either over time or in an instant. What I've noted here is only a bare beginning.

  • Connecting the battery cables backwards, reversing the polarity of the entire electrical system.
  • Using the wrong parts for the System and Fuel Relays. The proper relays have a diode to prevent spikey voltage from the coil discharge from reaching the ECU.
  • Shorts in the wiring harness.
  • Failed system or fuel pump relays.

Component degradation due to environmental causes.

The guts of an ECU is a circuit board populated with digital and analog electrical devices. Some of these are encased in a protective rubbery layer, and others are not.

  • Over time moisture intrudes and corrodes the components themselves or the electrical leads.
  • For a brief period of time, there were quality issues in the manufacture of Bosch ECUs, particularly in those destined for 1989 models of Saab. Units of that era were prone to cold solder joints, and failure of the leads to the fuel pump relay.

When you are looking at a car that won't start, or won't run properly, you have to consider the following, and respond accordingly.

  • Is the ECU actually at fault?
  • If the ECU is faulty, did it die because of
    1. progressive internal decay, or
    2. an insult external to the vehicle, or
    3. an insult internal to the vehicle.

Sorting this out is best done if you have a clear idea of what happened in the life of the vehicle just prior to failure. For example, if the muffler shop people tell you your car won't start, or if your kid brother tells you the car won't start even though they jumped it, then you are looking for major damage, including a dead ECU, caused by stupidity. In that case, you must take care to verify that the error didn't damage a relay, or wiring, or create an additional fault which can also lead to ECU damage. Casually swapping parts invites more trouble. Or, if the car was running fine, you shut it off, and it wouldn't restart, that suggests internal degradation of the ECU. And in that case swapping in a new ECU is ok (but first make sure you have the proper system and fuel relays.)

There are diagnostic sequences in Bentley for determining the cause of no-start situations. Its important to follow them because 1) they will identify the faults which are cheaper to fix than an ECU, and 2) they will identify faults which lead to (or will lead to) ECU failure.