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The Common Types Of Failure In ECU

We will divide this training into three sections to make it easier for the reader. The sections will be as follows

Part One : the common types of failure in ECU

Part tow : The methods of programming in ECU

Part Three : programming the ECU by diagnostic port OBD II

Before starting, we will provide a comprehensive and brief introduction to understand some of the elements and how they work in ECU.

Automotive computer

Receives input data.

Perform calculations.

Sends output signals.

Stores information.

Its internal components are:

1. Input interface.

Receives signals from sensors.

Converts digital signals for the CPU.

2. Output interface.

Convert digital signals into applicable form for sensor.

3. Central processing unit.

Receive the data.

Do the calculations.

Respond according to the program.

Send responses.

4. Storage memory.

Read-only memory (ROM)

Stores most of the basic information

Can't be changed

Programmable read-only memory (PROM)

Can be changed

Sometimes called Flash E-PROM

Random access memory (RAM)

Data is temporary

Called when required

It may be overwritten.

5. Electrical components.

An ECU is basically made of hardware and software (firmware). The hardware is made of various electronic components on a PCB. The most important component is a chip microcontroller together with an EPROM or a Flash memory chip. The software (firmware) is a lower level set of codes that runs on the microcontroller.

The ECU is characterized by:

Several analog and digital I/O lines (high and low power).

Power interface/control device.

Different communication protocols (CAN, KWP-2000, etc.).

large matrix switching for signals high and low power.

Smart interface adapters communication (standard or customized).

Automatic equipment recognition and enable software sequence.

Power device simulation


Automotive computer failures are divided into two:

Physical failures.

Logical failures.

Physical Failures.

Caution: Computer components automotive are extremely sensitive to electro-static discharges (ESD). Before handle any component, make sure wear an anti-static bracelet or touch some grounded object, such as a metal object, to eliminate any remaining static charge on the body.

The static electricity of the human body, can irreversibly damage circuits automotive computer integrations. Take precautions before uncovering the ECU.

In the ECU we can find 5 types of failures physical ones which have repair, the other faults possible is a damaged microcontroller, but this components are custom made by the manufacturer with specific values, which is Impossible to get a replacement in any store electronics. Fortunately these components they are the last ones to be damaged.

The 4 faults that we find in a computer automotive are the following.

1.Cracked or corroded weld point by rust. (This is a very common fault is 60% of Damaged ECUs are due to this cause.

2.Burnt track

3.Damaged transistor

4.Capacitor failures

5.Micro-processor Failure

1.Cracked or corroded weld point by rust

Although the cracks at the release points are hardly noticeable, they must be repaired since they cause failure. It also happens with points that show corrosion due to rust. This type of anomaly is the cause of 60% of ECU failures.

But there is a question that comes to mind regarding this type of damage.  Are there times when you can tell if the ECU has this type of failure without needing to uncover it?. The answer is. Yes, there are times when we can know, when presents one of the following two cases:

1.When there are times when the vehicle starts, and there are also other times it does not to start for no reason .

2.When a fault occurs on the vehicle and then disappear.

Only in these two cases can we be 100% sure. That the problem lies in welding points that they are cracked. this does not mean that this type of failures occur only in the two above cases. Which it means that only in those two case we can be sure that the problem is a welding point cracked or with rust corrosion. For all other cases which they may exist, they necessarily you must perform a visual inspection.

Put the ECU on your table to open it, then remove screws and the retaining clips.

the common types of failure in ECU

The weak point of ECUs is that the solder joints of the connectors soldered to the board are prone to cracking. This is the most common failure that occurs.

The image on the left shows in this case a transistor that presents this type of failure.


This image shows the solder joints on the back of plate.

solder joints

All three weld points are cracked. It may not seem like much, but this type of anomaly causes a momentary loss of connection. Corrosion accumulates in these cracks. To verify if an ECU has this type of failure, a visual inspection must necessarily be carried out.

solder joints

A good repair involves heating the cracked solder, removing the old solder, and then re-soldering it.

the cracked solderthe cracked solder

2.Burned track.


This represents a fault, which is usually solved with him making external bridges, in this case the solution would be to weld a fine wire on the track burnt. In this case they must also replace all capacitors electrolytics.

3.Damaged transistor.

First case:

Damaged output transistor.

Commonly the output transistors are the ones that are damaged, these transistors are located in line, in which a heat sink covers them. Pry each side a little at a time to remove it. Do not try to remove the insulation at the same time as it may break.

You can now safely remove the insulation.

Damaged output transistor

Here we have the output transistors, we proceed to verify each one.

Damaged output transistor

We proceed to check each transistor, all of them must return equal values. For checked we proceed as follows.

Damaged output transistor

First we check Base-Emitter:

TEST output transistor

Then we check Base collector:

TEST output transistor

To verify which transistor is damaged, the values of each transistor are measured, and the one that gives a different reading is the defective transistor.

Damaged 5v regulator transistor:

Damaged 5v regulator transistor:

The procedure to verify if this type of transistor is damaged, it is different from the case former.

This type of computer transistors automotive are damaged when they have the following characteristics:

When it reads 0 Zero Ohms of a terminal to another. (It is shorted between both terminals).

When it does not mark continuity from Base to Emitter or Base to Collector on the X10 scale or from Emitter to Collector on the X10K scale (in analog tester) (is Open Base to Emitter or Collector or from Emitter to Collector).

When it marks continuity from Emitter to Collector on the X10 scale. (Short-circuited from Emitter to Collector).

When it does not mark continuity from Base to Emitter or from Base to Collector on a Digital tester on the scale to measure Diodes. (Which is the scale used to measure Transistors in Digital testers).

It cannot mark both ways, that is, if the transistor marks continuity with the negative test lead on one terminal and the positive test lead on another terminal cannot indicate continuity if we exchange the test leads. Because you can only score in one way, in one direction so to speak.


The Transistor cannot mark continuity anyway, nor can it not mark at all. The normal thing is From Base to Emitter, From Base to Collector on the X10 scale and from Collector to Emitter on the X10K scale. (I will explain it below.) You cannot mark from Emitter to Collector on the X10 scale. Otherwise it is damaged. (open from Emitter to Collector.)


When we talk about continuity, it is not refers to Zero Ohms but to a measurement more or less, neither too low nor too high.

Normal Reading

Normal reading is when we place a test probe at the Base of the Transistor and the other test lead is in the Emitter and the Collector (first one and then the other.) and gives us a normal reading, that is, it does not measure or zero or infinity, neither much nor little, etc. And if we exchange the test leads, no mark nothing.

4.Capacitor failures

Note that the title NOT ONLY refers to physically damaged capacitors, in power failures capacitors we find two types:

1.Failures due to UNDAMAGED Capacitors, but that have lost capacity.

2.Failures of physically damaged capacitors.

Failures due to lost capacitors ability

Always be wary of electrolytic capacitors, especially if the ECU is more than 7 years old. When you notice any burnt traces on the ECU printed circuit, proceed to replace all the electrolytic capacitors. (ONLY ELECTROLYTIC CAPACITORS). Since it is almost certain that they have lost capacity, and when not, having been directly responsible for the failure, for little money we ensure its correct functioning.

Even if the capacitor IS NOT DAMAGED, it You must proceed to measure your capacity, in this way make sure it works perfectly.

Electrolytic capacitors have a disadvantage. They tend to escape with age when exposed to energy and heat cycles. Electrolyte leakage is very harmful to ECU boards. It can actually eat away at the copper scraps and over time short out the board. When that happens, the ECU will either stop working completely or act strangely. As the capacitor leaks, it also loses its filtering properties, possibly allowing harmful spikes in the ECU.


Carefully observe the polarity of the capacitor before removing it, to replace it exactly the same. Many plates are poorly screen-printed and have the polarity reversed. TAKE PRECAUTIONS IN THIS CASE.

Failures due to physically damaged capacitors Damaged capacitors are very easy to recognize, they are inflated at the top. Like the example in the figure.

physically damaged capacitors

Microprocessor failure:

The way to verify if the microprocessor is damaged is through the diagnostic interface, in this case we are using VAG_COM.

Microprocessor failure:

If the ecu manages to communicate with the VAG-COM it means that the micro-processor is working perfectly, in case it can communicate then it is damaged. A computer with a damaged microprocessor is very difficult to repair since this part is custom designed by the manufacturer, which makes it impossible. look for a replacement.

Important observation:

That there is communication between the vag com and the ECU does not mean that the ECU is in good condition, it only means that the microprocessor is working well, therefore we rule out failure in the microprocessor.

Logical failures

These types of faults are easier to repair and do not require unclogging the ECU. We will start with a small introduction about automotive computer communication protocols.

One more of the applications and capabilities of computers is communication, and taking advantage of this characteristic, a communication standard or protocol called OBD. (On Board Diagnostic).

OBD II is a standard that seeks to reduce the levels of pollution produced by motor vehicles. OBD II is therefore not a electronic injection system, but a set of standardizations that seek to facilitate the diagnosis of vehicles. The OBD II standard is very extensive and is associated with other standards such as SAE and ISO, so we are going to mention only the most interesting parts such as:


It is a 16-pin type:

It must be located in the driver's area, under the instrument panel.

Pin Description

2 - SAE VPW/PWM Communication

4 - Vehicle MASS

5 - MASSA Signal

7 - ISO 9141-2 Communication (K Line)

10 - PWM communication

15 - ISO 9141-2 Communication (Line L)



There are basically three types of communication that can be used and are chosen by the manufacturer: SAE VPW - variable pulse width modulation SAE PWM - pulse width modulation ISO 9141-2 - CAN serial communication. These communication systems obey called request-response patterns "communication protocol". Were detected the following patterns used by the assemblers:






The format of the defect codes must have the following presentation:


In addition to fault codes, OBD II allows the verification of various readings in real time such as: RPM, lambda probe, engine temperature, engine load, MAP, vehicle speed, MAF, ignition advance, air temperature, sensors after the catalyst, etc.


The primary objective of OBD-II is to help quickly and effectively detect a failure in the injection system, with the sole objective of minimizing vehicle gas emissions. When the system has some failure in its operation, gas emissions increase beyond the limits, and this is the mission of the standard. OBD-II establishes the forms and procedures for detecting these faults.

For faults check and correct software faults we use a scanner. We proceed to connect the scanner with the vehicle. We verify that the ignition LED is on. The scanner is made up of its connector.

Likewise, a software that is responsible for detect the different fault codes and delete them.

And they connect via Bluetooth to our personal computer.

Then we open the switch

Once the switch is open we can verify the data on the computer.

The software analyzes the vehicle's computer and identifies the fault codes. The same program also erases the fault codes that are generated, thus repairing the vehicle's computer.





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